electric shock
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being
careful
Recently in All in a Life, Harry Leeming discussed electrical
safety. This has obviously struck a chord with readers and, in
this article, we’ll answer some of the questions you’ve asked.
I
From early research into the American electric chair
there is reason to suppose that there is a critical
voltage is required to delete life quickly. Of course, an
extremely large voltage, such as that on the grid
system or associated with a primary lightning strike,
has the possibility of dispatching the recipient from
this world in an instant.
There’s swift vaporisation of moisture and, since we
are composed mostly of water salts, the probability of
surviving is remote. The same thing happens when a
tree is struck by lightning. Conductivity in summer is
higher as gallons of water are lifted to the top from
the root by capillary attraction. Thus the high electric
potential of the strike can return an incredibly high
current from earth, which changes the sap into steam
in the blink of an eye, leaving the so-called ‘lightning
tree’ of well seasoned wood.
School Experiments
f you are involved in electronics, you will probably have
experienced an electric shock of some kind. Some have
experienced this many times! The principle is that a DC shock
tends to throw the person away from the source, quickly
severing the connection, while an AC shock causes the muscles to
tighten so that the grip is strengthened, it then being very difficult, if
not impossible, to sever the contact by force of will.
All electrical shocks at 240V can cause death, but fortunately the
majority of shocks are limited to safety levels by pure good luck.
Probably the resistance of sound footware and of flooringing material
saves many lives every year.
The majority of electrical fatalities involving users of electricity occur
as shocks to earth. Electrical shocks experienced by unqualified people
are nearly always the result of irresponsible meddling and it is not
possible to provide a means of saving them from the consequence of
their follies. The user of electricity is frequently in contact with earth
through flooring materials or with adjacent earthing metal. If the frame
of a machine becomes ‘live’ or if the enclosure of a plug, terminal box or
the sheathing of a flexible cable becomes damaged, only one contact
with the ‘live’ metal work will put the user’s life at risk.
Have you ever measured the DC resistance of your
body? It’s an experiment that seems to capture the
imagination of just about every physics student. This can range over
about 10 to 20kΩ, using a simple ohmmeter, depending on the contact
area, how tight the electrodes are squeezed in the hands, moisture
content, ambient temperature and so forth.
However, this is certainly not the real body-current-determining value.
The human, body represents a multiplicity of conducting paths - some in
parallel with capacitance and others passing through the heart. A net
impedance rather than pure resistance is thus represented. This means
that the current is frequency-dependent, while the current through the
heart would need the laws of Kirchoff to resolve! A simplistic version of
the situation is shown in Fig. 1.
Current flow is also influenced by the skin contact voltage, so the
simple ohmmeter measurement is barely relevant. Research has shown
that for contact areas between 500 and 1000mm2 and potentials
between 50V and 1kV, the impedance of the vast majority of
human bodies at 50Hz ranges between 1.5 and 4.3kΩ.
So, taking into account the various capacitive
elements you could end up with an
average of 2kΩ up to 100Hz and
1.75kΩ from 100Hz to 1MHz.
So, how much current can
the average heart accommodate
before misfiring (fibrillation)?
There’s no straight answer for
that question as it depends on
the nature of the supply, area
of contact, condition of heart
and the like, at the time of
contact. We’ve all read
42
April 2005 - Radio Active
stories where something as low as 100V has resulted in death, equally
where a person has survived near lighting strikes.
The Danger
The danger to a person depends mainly on the magnitude and duration
of the current flow. However, the relationship between the voltage
source touched by the person and the current that passes through that
person is not linear because the impedance of the human body varies
due to a number of factors. Different part of the body, such as the skin,
blood, muscles and other tissues and joints, present a certain impedance
composed of resistive and capacitive components. The values of these
impedances depend on several factors - current path, touch-voltage,
duration of the current flow, frequency of the touch-voltage, moisure of
the skin and the surface area of contact.
We’ve already mentioned that the average impedance of a human is
around 2kΩ. However, this impedance will be lower for DC values of
voltage. Results from experiments have shown that a minimum current
flow of just 0.5mA can be detected by the human body. the threshold of
‘let go’ is about 10mA and the restriction of breathing begins at about
30mA. Fortunately, ‘no-let-go’ intensities have no permanent harmful
effect, providing they do not rise above 20mA or unless they are
sustained for very long periods. However, they do cause intense pain
and are usually very frightening.
A Rescue
The majority of incidents in cases of electric shock all have means of
escape, but usually panic prevents the victim from seeing them and can,
sometimes, turn a serious situation into a tragic one. Pre-knowledge of
the possible means of escape can greatly reduce the tendency to panic.
electric shock
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Muscular restriction seldom incapacitates all four limbs and the one
remaining useful - probably an arm - should be used to break contact.
Respiration is unlikely to be affected in the early stages and the victim
can shout for help. Anyone involved in such a rescue should take
particular care that they do not become a second victim. The rescuer
should switch off the supply without delay. If this cannot be done, the
victim should be dragged or knocked from contact, taking care to avoid
touching their bare skin and, of course, live metalwork.
Un consciousness from loss of respiration will not occur until
breathing has been stopped for about three minutes and it can usually be
restarted by artifical respiration, which should be given immediately and
be continued until the patient recovers or, sadly, is pronounced dead by
a doctor.
An RCCB
The safety device used to detect leakage to earth and then to disconnect
the.supply is a Residual Current-operated Circuit Breaker, RCCB, which
is sometimes referred to as an ELCB, Earth Leakage Circuit Breaker.
The RCCB relies on the principal that in a fault free electrical system
there is no leakage to earth of the live or neutral conductors. The
current leaving the RCCB through the live conductor will return to it by
the neutral.
In the event of a leakage to earth, some of the current by-passes the
neutral conductor and returns to the supply source through any earth
continuity that may be present. The live and neutral cables, which carry
the load current, pass through a sensing core. The effective sum of the
live and neutral currents passing through the core must be zero and
therefore produce no magnetic field. When leakage to earth occurs, one
Lessons
cable will carry more current than the other and the magnetic field
So what lessons can be learnt?
produced by one core will not
Skin R
The disconnection of the supply
now cancel out the magnetic
Green/Yellow
is important for both the victim
field produced by the other
to Earth
and rescuer. A switch that
core. A potential difference is
isolates the complete supply,
induced into the secondary
excluding lighting, to the shack
coil wound round the sensing
Blue to Neutral
should be located at the entrance
coil tha will energise a
to the room. A suitable type of
tripping relay, which
emergency switch could be a
disconnects the supply from
number of emergency stop
the load.
buttons, well placed, that will
The operating time of the
disconnect all the shack supplies
RCCB varies between 30 and
Skin R
excluding lighting when any one
50ms. The margin of safety
of them is pressed. The stop
these devices provide are
RT2701
buttons should be the type that
some 100 times greater. The
latch in position when pressed
rapid clearance time prevents
and need to be turned to release.
any significant rise in shock
In industry, you would expect
intensity above 240mA.
Skin R
RT2700
Skin R
there to be a clear working space
As these devices are
wherever work is carried out near
Fig. 1: Elementary Impression of the resistive and capacitive elements current operated, they
live conductors, so why is the shack of the human body, the whole forming a complex Impedance
provide a further benefit, by
any different? A minimum
helping to reduce fire hazard
passageway of one metre should be maintained. If you have a desk and
due to live to earth faults. With no earth leakage protection, it is possible
workbench or table that form a corridor, then there should be 1.5m
for a large current to flow to earth due to a fault, yet if the circuit is
between the two.
protected only by a fuse or conventional circuit breaker, no current
In industry there is a long-standing practice that if someone is working
overload will be detected. The power dissipated as a result of the leakage
on live equipment then they should be accompanied. This is because
current can be the cause of a fire. A circuit protected by an RCCB
anyone, however experienced, can make a mistake and, in the case of
would have disconnected the supply long before the leakage current can
electric shock, immediate assistance can be a lifesaver. This is not really
start a fire.
practical as far as the shack is concerned. However, it does suggest that
Although the RCCB device seems ideal to improve the electrical
it would be wiser to have the shack inside the house in close proximity
safety within the shack, the device may prove counter-productive in
to members of your family who could give assistance. But, care must be
giving the radio user a false sense of security since it provides no
taken to ensure that members of your family are prevented from
protection against electrical shock from the secondary side of any
receiving an accidental electric shock from your radio equipment.
double-wound mains transformer. This includes the HT supplies to the
high power valve amplifier that can often be 1, 2 or 3 thousand volts.
One of the important parameters of the RCCB device is the trip time.
A Shock to the Heart
How can one ensure that the RCCB is operating correctly? It should be
The most likely cause of death resulting from an electrical shock is the
tested regularly. This does not imply testing by pressing the internal trip
effect the shock has on the human heart. The effect of current flow on
button, but testing by a purpose-made instrument. There are a number
the heart function will depend on the physiological parameters as well as
of manufacturers producing equipment that will test not only the trip
the electrical parameters such as duration and pathway of the current
time but also the trip current. You should be aware that if you have
flow. The electrical current that interferes with the heart function causes
incorporated a mains RF filter into the shack wiring
ventricular fibrillation. This is a condition in which the rhthym of the
circuitry then you can expect random
heart valves is upset by the effect of signal from an external source
tripping of the RCCB. It is most
acting at random. It can only be corrected when the necessary skill and
likely that capacitors
equipment are immediately available. The supply of 50Hz is dangerous
forming part of the
because it is sufficiently near the natural heart rhthym to seriously effect
filter will be
the function of the heart.
connected between
High frequency currents and DC are free from fibrillation effects,
live and neutral
although they can cause damage to the nervous system with in the body
and live to earth.
by overheating. Extremely large shocks can stop the heart completely.
More next
Ventricular fibrillation is unlikely to occur below 50mA. Remember ‘It’s
month.
■
volts that jolt but amps that kill’. The duration of the shock and its
intensity are equally important in relation to its fibrillation effects.
Therefore it is important if ventricular fibrillation is to be avoided that
some form of protective device should be fitted. Preferably one that is
not only current sensitive but that will disconnect the supply within a
given time of earth leakage being detected.
April 2005 - Radio Active
43
42 being careful (Final)
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being
careful
We continue with our look at electrical safety as we start
learning about the wiring in the radio shack
I
n planning the electrical wiring of the shack, consideration should
be given to providing devices that will protect the electrical
installation against damage resulting from a current overload.
This protection could have been provided in the past by inserting
suitably rated fuses into the various branches of the electrical
installation. The protection to over-current provided by a fuse relies on
the wire forming the fuse becoming hot and eventually melting to break
the circuit as a result of over-current.
Way back in 1889, Thomas Edison defined a fuse as a small safety wire
that became heated and melts away, breaking the overloaded circuit.
As a general rule, a fuse will not blow instantaneously unless
subjected to an overload current many times the fuse rating. For
example, for a 6A fuse to blow within 10ms it needs an overload current
of approximately 72A, which is 12 times the rating of the fuse and it is
not always possible to achive a high fault current due to the short circuit
impedance of the total circuit.
The prospective fault current at a given point is the maximum steady
state RMA current that would flow if all the active conductors were
solidly bolted together at the point in question. The current is thus the
maximum fault current that may have to be interrupted by the protective
device. In practice, the actual fault current is usually much less than the
prospective fault current, due to the current limiting effect of the fault
resistance and of the cabling from the protective device to the point of
the fault. Rarely does a zero impedance fault occur immediately adjacent
to the protective device.
The main factors that limit the prospective current are the size of the
supply transformer and the impedance of the cabling, switch gear, joints,
etc., from the sub-station transformer to the point of application. The
Supply Authorities have an statutary obligation to provide overall current
protection. In the majority of domestic premises, the Supply Authority
cut-out will contain a 60A fuse, which will limit the over-current to an
acceptable level. The disconnection time for that cut-out should be -.5s,
which will require 400A or 700A for a disconnection time of 0.4s.
Under fault conditions, damage can be done to the installation and
associated equipment because the amount of energy that passes before
the fault current is completely intrerrupted. The total energy let through
depends on the value of the current and the time for which it flows.
Miniature Circuit Breakers
The disconnection time can be reduced if an electro-mechanical
protection device is used instead of a fuse. The electro-mechanical
device is called a Miniature Circuit Breaker (MCB). The MCB consists of
a length of thermo metal, a solenoid and a trip bar mechanism. When
the overload current just exceeds the rating of the MCB, the current
flowing through a length of thermo-metal will begin to deflect it in
response to the heat generated. The thermo-metal will begin to move
against the trip bar, releasing the trip mechanism. If the overload current
is high, due to a short circuit condition, the current flowing through the
MCB will also pass through a solenoid that will pull-in a hinged action
plunger, forcibly separating the contacts and simultaneously releasing the
trip mechanism.
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May 2005- Radio Active
In both situations of overload, the moving contact
moves away from the fixed contact and an arc will be
established between the contacts. The arc will run
along the arc runner to the arc chamber where it will
be split up between the plates and extinguished. The
low inertia and consequent high speed of the moving
contact has a limiting effect on the flow of fault
current.
The rapid development of the arc together with its
associated extinction in the arc chamber gives a
typical operating time of 3.5 to 5ms. The rapid speed
at which the contacts are parted prevents the fault
current from reaching its peak value. The high speed
current limiting action of the MCB ensures that the
energy let through and any subsequent damage is
minimised.
Installing MCBs
To cater for the different environments that an MCB
may encounter, different operating characteristics
have been devised to meet the majority of different
situations. For example, it would be unwise to install
an MCB with a sensitive characteristic in a circuit,
which is likely to experience heavy switching surges.
The different classifications of MCB are:
Type 1: Those having a magnetic operation over the range 2.7 to 4
times the rated current.
Type 2: Those having their band of magnetic operation over the range
4 to 7 times the rated current.
Type 3: Those having their magnetic operation between 7 and 10
times the rated current.
These values are such that in each case the current flowing ensures
that the MCB will trip within 100ms. The Type 1 characteristic is best
suited for use on loads with little or no switching surges, as occurs in
domestic application and the amateur radio shack! In addition, a Type 1
MCB, when compared with the characteristic of fuses, will give indirect
shock risk protection for high values of loop impedance.
The Type 2 characteristic is best suited for general
commercial/industrial applications combining maximum usage with
closest protection. Although this type of MCB gives a similar degree of
indirect shock risk protection to that given by certain fuses, the closer
protection it affords on smaller overloads combined with the slower
operation on heavier faults enables it to offer better protection with less
likelihood of nuisance tripping.
Regulations require that the MCB, which is an over-current protective
device, should be capable of making and breaking any over-current up
to and including the prospective short circuit current at the point where
the device is installed. The trip-free mechanism will be designed to
ensure that the breaker will, on experiencing the over-current again,
open and therefore safely isolate the fault again.
Typical Installation
In a typical installation there will inevitably be other devices between the
MCB and the source of supply due to the methods of breaking down the
supply for the purposes of distribution. These other protective devices
may be larger MCBs or fuses of either the re-wireable or cartridge type.
In the typical situation, the ring-main circuit will be protected by a fuse
or MCB at the point of distribution.
When designing a distribution system it will be necessary to consider
42 being careful (Final)
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Page 43
the problems of ensuring effective discrimination between all the
protective devices. Ideally, the protective devices in an installation should
be so graded that, when a fault occurs, only the protective device
nearest the fault should operate. The other devices should remain intact
and should continue supplying the other healthy circuits.
Back up Protection
In the situation where the prospective short circuit current might exceed
the breaking capacity of the MCB, back-up protection will be required.
If the prospective short circuit current is greater than 6000A, then the
MCB. will equire back-up protection by a suitable fuse. At the maximum
fault level, the fuse may operate first. In most cases, it will be the MCB
that operates before the fuse and disconnects the circuit. In practice it is
usually advisable to aim at the back-up fuse taking over at a fault current
level not exceeding 70 per cent of the MCB’s capacity. However, the
value of the back-up fuse must not be too low such as to lose
discrimination between the two protective devices.
Power Distribution
Before starting on the design of a power distribution system for the
radio shack, it is wise to first of all establish the size of the Supply
Authority fuse. If this information is not readily available, then it is
sensible to base the installation design on the premise of a 100A fuse
being used. The existing domestic electrical installation is likely to have a
main distribution consumer unit containing a number of fuses or MCBs
that distribute the domestic supply between the lighting and power
circuits. If there is no spare position for an MCB or a fuse, then the
consumer unit will need to be changed for a larger unit. This sort of
work must be carried out by a technically competent person, and
reconnection of the Supply made the Electriciy Board.
The total electrical energy demand likely to be required for the radio
shack should not exceed 10kW, unless you are planning some DX
activity which is outside the terms and conditions of your licence! For
supply demands below 10kW, the electrical feed to the radioshack can
be provided .by a cable containing a conductor size of 6mm2. Probably
the most popular type of cable used in domestic and some commercial
wiring is known as 6mm2 twin and earth, which is protected by a grey
pvc sheath. The current carrying capacity of this cable is 40A if the
cable is enclosed and unable to freely dissipate heat losses.
If the cable is clipped directly to the surface of walls and is not
enclosed at any point along its length, the current capacity of the cable
is increased to 46A. The cable can be terminated in the main consumer
unit by either a fuse or MCB. The value of the MCB should be such that
it will protect the feed cable, supply the required load and operate if a
short circuit condition should occur on the load side of the MCB without
any other protective device operating on the supply side of the MCB.
The 6mm2 cable will be adequately protected by the 30A MCB.
Manual Isolation of the Supply
The shack end of the supply cable needs to be connected to some
device, which will quickly disconnect the supply manually in an
emergency. From the previous discussion on electrical safety, it is
important that the rescuer in an emergency can disconnect the supply
easily and quickly inside the radio shack. It is recommended that an
isolating switch should be positioned within easy reach by the entrance
to the radio shack together with a number of emergency stop buttons
that should be strategically placed around the shack. These should
disconnect the power supply to the radio shack, excluding lighting, if any
one of the stop buttons is pressed. The stop button can be connected to
a contactor that will have its contacts normally open when the contactor
coil is not energised.
You should be able to find a triple-pole and neutral MCB that includes
a shunt trip mechanism. The shunt trip must be connected such that the
coil is de-energised once the MCB has operated. The shunt trip coil is
suitable for use on the normal mains supply. They are available in a
range with different current ratings varying from 0.75A up to 60A. In
this application a 20A MCB will be suitable and would also protect the
power distribution within the radio shack. The MCB can be housed
within a standard surface mounted enclosure.
An alternative method is to use a double-pole MCB containing a
shunt trip for external tripping. Shunt trips are available for single-pole,
double-pole and triple-pole breakers. It may also be advisable to fix a
suitable notice above the isolating switch appropriately labelled. The
notice should advise visitors to the shack, the location of the supply
isolating switch. The various thump switches can be located in
appropriate positions underneath the benches, but in view of the
operator. Try and avoid locations where the knees are likely to come
into contact with these thump switches and accidentally isolate the
supply. This sort of situation often leads to the switches being tampered
with to prevent accidental isolation and so they don’t operate in an
emergency when required. The type of thump switch required will
depend upon the type of isolating device you choose to use. A wide
range of thump switches can be purchased over-the-counter direct from
your local electrical wholesale dealer. The output from the MCB with the
shunt coil should be fed directly into the radio shack consumer unit
which will contain the RCCB.
Shack RCCB
There are several companies that manufacture consumer units that can
incorporate an RCCB protective device. In some consumer units, the
RCCB occupies the space normally occupied by two MCB modules. The
RCCB can be supplied in a range of different current ratings from as
little as 10mA up to 300mA earth leakage trip currents. The consumer
unit will accept the MCB that is supplied in a range of different singlepole overload tripping currents ranging from 5A to 30A.
Another company also manufacture a consumer unit that will house
an RCCB that occupies the room taken by three MCBs. The RCCB can
be supplied in a range of earth leakage trip currents varying from 10mA
up to 300rnA. The individual MCBs are manufactured with a range of
overload currents varying from 6A up to 40A. To increase the level of
safety within the radio shack it would be advisable that the lowest value
of RCCB should be used. However, this may prove counter-productive
with an increase in nuisance tripping because of high values of leakage
current to earth that may occur as a result of old mains transformers
usually associated with pre-war and war-time manufacturer
communications equipment.
If you think that this is likely to create a problem within your radio
shack, the solution could be to fit a 30mA RCCB in place of the 10mA
RCCB. As discussed earlier, a 30mA RCCB will provide a high degree
of protection against electrocution in an accidental radio shack hazard
situation. The current flowing through a human body could vary
between 30 and 250mA, depending on the resistance of the human
body and the voltage across it. To be within Zone 2, mentioned last
month, it is necessary for the RCCB to operate within 50ms at 240mA
and 150ms at 80mA. Both these conditions are satisfied by a 30mA
RCCB. If 10mA RCCB is used instead, further protection against the
likelihood of ventricular fibrillation will be provided.
■
May 2005 - Radio Active
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being
careful
L
Shack consumer unit
RT2788
We continue with our look at electrical safety as we finish
the wiring in the radio shack
T
he size of the shack consumer unit will depend of the system
of power distribution you adopt for the radio shack. It is wise
to consider what equipment needs to be connected to the
Fig. 1.
power distribution system. The last thing you want is not to have
sufficient power sockets and be continuously swapping around plugs to
connect pieces of equipment to the supply. Another consideration is the
possibility of segregating the supply to various pieces of electrical
equipment. For example, the amateur radio equipment can be fed from a
different part of the supply to that for the test equipment. This will help
to split up the electrical load and permit you to use lower values of
MCBs, which in turn will improve the protection against overcurrent.
The simplest form of power distribution that you are likely to
encounter is the ring-main circuit, which is shown in Fig. 1, where an
unlimited number of socket outlets can be provided. According to
Regulations, the maximum floor area served by the ring main should
not exceed 100m2. It is unlikely that the average radio hobbyist will
have a radio room exceeding this! The over-current protection device
must not exceed a rating of 30 or 32A. The rating of the protective
over-current MCB feeding the ring main should be less than 20A to
ensure that it operates before any other protective device that has
been installed in the supply feed to the radio shack.
The Regulation also recommend that a 2.5mm2 copper conductor
of either rubber or pvc insulation should be used to connect the 13A
sockets in the form of a ring. The most popular cable available from
your local wholesaler is the 2.5mm2 grey coloured pvc covered tin and
earth. Both live ends of the cable are connected into the MCB inside
the radio shack consumer box. The neutral conductors are connected
into the common neutral block found inside the consumer unit. It may
be advisable to provide more than one ring main within the radio
shack if you should decide to split up the supply distribution into
essential and non-essential supplies.
Radial System
An alternative way of distributing the power around the radio shack is
by using a radial system as shown in Fig. 2. Again you can use
2.5mm2 pvc covered twin and earth. An unlimited number of socket
outlets can be used on each spur. However, the over-current
protective device must not exceed 20A per spur. Each spur must not
supply a floor area greater then 20m2. This system of power
distribution provides the flexibility to split up the load into individual
spurs that can feed different types of equipment. For example, the
main station transmitter equipment can be fed on one spur, while the
test equipment is fed by another. Any other station equipment, such
as the VHF rig can be fed by a third spur.
You will find that you will never have too many 13A sockets. When
planning the layout of the supply distribution always provide more
13A sockets than you think you will require. The size of the over
current protective device can be reduced to 5A when supplying small
electronic equipment such as test equipment and low power
transmitting equipment. It is also wise to remember that when
36
June 2005- Radio Active
determining the size of the consumer unit to use,
allow space for a small number of spare MCB
modules. This will provide you with the flexibility
later to expand the supply distribution. The gaps left
by the absence of the spare MCB modules can be
covered by a small blanking plate supplied by the
manufacturer.
Isolated Supply
Both the power distribution systems described rely on
being fed directly from the Supply Authority. This will
result in one leg of the supply (the neutral) being at or near earth
potential, while the other leg of the supply (the live) will be at full mains
potential, 240V. To reduce the chances of receiving a fatal electric
shock, the mains potential on the live conductor can be reduced by
using an isolation transformer as shown in the circuit Fig. 3. The
isolating transformer T1 must have a centre tapping secondary
winding. The size of the transformer will depend upon the supply
requirements for the radio shack. The transformer can be supplied
contained within a protective metal box for either floor or wall
mounting.
The size of the MCB with the shunt trip will be determined by the
size of the isolating transformer being used. Remember that the size of
the MCB must be able to protect the transformer form accidental
damage should the secondary winding become short circuited. The
secondary of the transformer should be fed to the 30mA RCCB that
can also be wall mounted, if required.
Earthing Arrangements
The earth connection provided within the radio shack is not only
important from the point of view of safety, but also from the earthing
on the radio frequency side. The earth connection provided by the
local Supply Authority cannot always be guaranteed as a perfect or
local earth. It is, therefore, advisable that a separate earth connection
should be made locally in close proximity to the radio shack. Your
local electrical wholesaler will be able to provide over-the-counter
suitable earthing rods. To obtain a reasonably good local earth, it will
be necessary to join together a number of 1.2m by 16mm2 rods as
shown in Fig. 4. Before driving the rods into the ground, ensure that
the rods are not likely to damage any hidden drains of buried cables.
Kango supply an attachment for their automatic hammer that will
make the job easier and will not damage the threaded end of the rods.
The Kango hammer and attachment can be hired on a daily basis
from your local plant hire company. Coupling pieces can also be
purchased for joining rods end-to-end. A special pointed piece can be
bought that can be screwed onto the front end of the first earth rod to
assist its movement into the ground. It is wise to attempt to drive as
many rods end to end into the ground to achieve a good earth.
Another special attachment is available that can be screwed onto
the end of the last earth rod to terminate the earth wire. It is
36 being careful June
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2:42 pm
Page 37
L
L
N
Shack consumer unit
20A
MCB
RT2789
T1
30mA
RCCB
N
RT2790
Shack consumer unit
Fig. 2.
Fig. 3.
Local earth to radio room
RT2791
Minimum 10mm 2 stranded
copper green pvc insulation
Conclusions
Whenever working in the radio shack, always exercise extreme care
when using mains operated equipment. Familiarity breeds contempt. It
has, in the past, been suggested that the use of RCCB safety devices
can create a false sense of security for the individual being protected
and can therefore indirectly cause accidents to occur through
carelessness. So, whenever using electrical equipment irrespective of
the protective devices user, always exercise extreme care.
When servicing live equipment always ensure that metal jewellery
such as rings and watch straps are removed before inserting the hands
inside the equipment. Wherever practicable, only use one hand to
service live equipment. The hobbies of amateur radio, CB and listening
have a good track record a far as safety is concerned. Let’s hope that
the introduction of safety devices into the radio shack power
distribution system will assist in the protection of the individual against
receiving a serious electrical shock and at the same time will also
protect the electrical equipment from being damaged as a result of an
electrical malfunction.
■
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recommended that the earth wire should be not less than 100mm2
stranded copper wire covered with green pvc insulation. The end of
the wire should either be connected to the centre tap of the isolating
transformer secondary or to the earth terminal provided within the
radio shack consumer unit. From the consumer unit other earth
connections can be made to the various pieces of transmitting and
receiving equipment.
From the point of view of providing a good antenna system it would
be beneficial to connect a counterpoise wire to the earth wire
connection on the top of the earth rod and spread the counterpoise
wire around garden attached to the boundary walls. Another important
point is to remember to cover the joint between the earth rod and
copper wire with grease to protect it against corrosion.
37