Chapter 8
Power
The majority of outlets to be used for power services are socket outlets of the types
described in Chapter 1. The various ways in which power circuits can be arranged have
been described in Chapter 5. It is usually found that as soon as more than two or three
socket outlets are to be supplied, it is more economical to serve them from ring circuits
than from radial ones. On ring circuits, only 13A BS 1363 socket outlets should be used
in order to ensure that the plug must be of the fused type and that the appliance and
flexible cable to it do not rely on the ring-circuit fuse for protection. There is little more
one can add about socket outlets and it remains in this chapter to say something about
connecting fixed appliances and larger equipment.
Fixed appliances of small ratings, by which we mean up to 3kW, can be served
through fused connection units from the ring mains serving the socket outlets in the same
area as the fixed appliance. A 3kW electric fire is one example of a fixed appliance which
might be supplied in this way. If the socket outlets in the area are on radial rather than
ring circuits then each fixed appliance must have a separate radial circuit of its own. It is
often convenient to supply equipment such as motorized valves on hot-water heating
systems, roof-mounted extractor fans in kitchens, tubular heaters in tank rooms and so on
by a separate circuit for each item or group of items. There is nothing wrong technically
with supplying them by a fused connection unit from an adjacent general-purpose ring
main provided the permanent load they put on the ring is taken into account in assessing
the number of socket outlets that can be permitted, but these items have a different
function from the general-purpose socket outlets and it is logical to serve them separately.
Separation by function can be an asset to maintenance; there should be no need to isolate
all the socket outlets in part of a building when work has to be done to a toilet extractor
fan. On the other hand, a separate circuit to one small piece of equipment may seem an
extravagance. No general rule can be made, and the designer must decide each
application on its particular circumstances.
Equipment larger than 3 or 4kW must in any case have a circuit for every individual
item. This applies to cookers, each of which must be connected through a cooker control
unit. One cooker control unit may control more than one appliance if they are in the same
room. This would apply to a hob and separate oven. A cooker with four hot-plates, a grill
and an oven can take 35A when everything in it is switched on, but rarely is it used in this
way. Consequently the IEE Guidance Note 1 and the IEE On-Site Guide suggest a
diversity factor to apply to domestic cooker circuits. In restaurant and school kitchens the
cookers are likely to be in full use for the greater part of the time. Cooker control units
are generally rated at 45A and if they and the cookers are to be properly protected, the
circuit fuse must not be greater than 45A. It follows that the circuit cannot serve anything
in addition to the cooker control unit without being overloaded.
Other large equipment is likely to consist of motors driving pumps and fans in plant
rooms and machine tools in workshops and factories. In the case of plant rooms each
machine is almost invariably on a circuit of its own. Having more than one motor on a
circuit would make it necessary to use very heavy cable and would in general be less
economic than using a larger amount of smaller cable. It would also be extremely
inconvenient to have several machines put out of action if one of them blows it fuse. This
is particularly the case when one machine is intended as a standby for another. Similarly
in factories it is usual to have each machine on a circuit of its own. In small and mediumsized
factories the most convenient wiring method is probably one using conduit and
trunking. In such places there is seldom any objection to installing conduit and trunking
on the surface of walls, and this is cheaper than burying it in the fabric of the building. It
also makes it quite easy to alter the wiring when new machines are installed or the factory
is rearranged. For the same reason, it is also better to run the wiring at high level under
the ceiling and drop to the machines than to run it within the floor.
In large factories, a busbar system is often used. Bare conductors enclosed in a casing
are run round the factory, preferably at high level, either on the walls or under the ceiling.
A switch fuse is connected to these conductors as close as possible to each machine, and
the connection from the switch fuse is taken through conduit or trunking to the machine.
Each machine is thus on its own circuit, but no sub-mains other than the busbars are
needed. The busbars must be protected by an adequate switch fuse at the intake. It is easy
to connect a new switch fuse at any point of the busbars and the electrical installation is
thus both convenient and flexible.
In small workshops, for example, metalwork and engineering rooms in secondary
schools, the machines used may be small enough to make it practicable to serve a number
of them from one ring circuit. Each machine is connected to the ring through a fused
isolator or through a switch fuse. The fuse is necessary to protect the final connection to
the machine, which is necessarily of a lower rating than the ring main, and to protect the
internal wiring of the machine that will also be of smaller cable than the ring main. The
cables of the ring main should be capable of carrying at least 70 per cent of the total
current taken by all the machines, and it will be found that this very soon restricts the size
of workshop that can be treated in this way.
It should be appreciated that everything that has been said about power circuits applies
equally to three-phase and single-phase circuits. Where three-phase machines are used
three or four cables, according to the system, plus an earth connection, are installed, and
distribution boards, isolators and circuit breakers are of the three phase-pattern, but the
general circuit arrangements are the same as for single-phase circuits.
All mechanical equipment requires maintenance, and all machines and equipment
must, therefore, be installed in such a way that maintenance is possible. One of the things
that has to be done before maintenance work is started is the turning off of the electricity
supply, and it must be possible to isolate each machine or group of machines. It has been
known to happen that an electrician has turned off an isolator in a switch room and gone
to work on a machine some way from that room, that someone else has come along later,
Power 129
not realized that anyone was working on the machine and has turned the isolator on
again. Not only has this happened, it has caused deaths. Consequently, most safety
regulations, especially The Electricity At Work Regulations, now require that there should
be an isolator within reach of the machine, or is lockable–in any case the isolation must
be secure. The intention is that no one can attempt to turn the supply on without the
person on the machine becoming aware of what is happening. For small machines, such
as roof extractor fans, connecting the machine to the wiring through a socket and plug
near the machine is a convenient and satisfactory way of providing local isolation. For
larger machines, a switch or isolator or disconnector as it is now known has to be
installed.
Power
The majority of outlets to be used for power services are socket outlets of the types
described in Chapter 1. The various ways in which power circuits can be arranged have
been described in Chapter 5. It is usually found that as soon as more than two or three
socket outlets are to be supplied, it is more economical to serve them from ring circuits
than from radial ones. On ring circuits, only 13A BS 1363 socket outlets should be used
in order to ensure that the plug must be of the fused type and that the appliance and
flexible cable to it do not rely on the ring-circuit fuse for protection. There is little more
one can add about socket outlets and it remains in this chapter to say something about
connecting fixed appliances and larger equipment.
Fixed appliances of small ratings, by which we mean up to 3kW, can be served
through fused connection units from the ring mains serving the socket outlets in the same
area as the fixed appliance. A 3kW electric fire is one example of a fixed appliance which
might be supplied in this way. If the socket outlets in the area are on radial rather than
ring circuits then each fixed appliance must have a separate radial circuit of its own. It is
often convenient to supply equipment such as motorized valves on hot-water heating
systems, roof-mounted extractor fans in kitchens, tubular heaters in tank rooms and so on
by a separate circuit for each item or group of items. There is nothing wrong technically
with supplying them by a fused connection unit from an adjacent general-purpose ring
main provided the permanent load they put on the ring is taken into account in assessing
the number of socket outlets that can be permitted, but these items have a different
function from the general-purpose socket outlets and it is logical to serve them separately.
Separation by function can be an asset to maintenance; there should be no need to isolate
all the socket outlets in part of a building when work has to be done to a toilet extractor
fan. On the other hand, a separate circuit to one small piece of equipment may seem an
extravagance. No general rule can be made, and the designer must decide each
application on its particular circumstances.
Equipment larger than 3 or 4kW must in any case have a circuit for every individual
item. This applies to cookers, each of which must be connected through a cooker control
unit. One cooker control unit may control more than one appliance if they are in the same
room. This would apply to a hob and separate oven. A cooker with four hot-plates, a grill
and an oven can take 35A when everything in it is switched on, but rarely is it used in this
way. Consequently the IEE Guidance Note 1 and the IEE On-Site Guide suggest a
diversity factor to apply to domestic cooker circuits. In restaurant and school kitchens the
cookers are likely to be in full use for the greater part of the time. Cooker control units
are generally rated at 45A and if they and the cookers are to be properly protected, the
circuit fuse must not be greater than 45A. It follows that the circuit cannot serve anything
in addition to the cooker control unit without being overloaded.
Other large equipment is likely to consist of motors driving pumps and fans in plant
rooms and machine tools in workshops and factories. In the case of plant rooms each
machine is almost invariably on a circuit of its own. Having more than one motor on a
circuit would make it necessary to use very heavy cable and would in general be less
economic than using a larger amount of smaller cable. It would also be extremely
inconvenient to have several machines put out of action if one of them blows it fuse. This
is particularly the case when one machine is intended as a standby for another. Similarly
in factories it is usual to have each machine on a circuit of its own. In small and mediumsized
factories the most convenient wiring method is probably one using conduit and
trunking. In such places there is seldom any objection to installing conduit and trunking
on the surface of walls, and this is cheaper than burying it in the fabric of the building. It
also makes it quite easy to alter the wiring when new machines are installed or the factory
is rearranged. For the same reason, it is also better to run the wiring at high level under
the ceiling and drop to the machines than to run it within the floor.
In large factories, a busbar system is often used. Bare conductors enclosed in a casing
are run round the factory, preferably at high level, either on the walls or under the ceiling.
A switch fuse is connected to these conductors as close as possible to each machine, and
the connection from the switch fuse is taken through conduit or trunking to the machine.
Each machine is thus on its own circuit, but no sub-mains other than the busbars are
needed. The busbars must be protected by an adequate switch fuse at the intake. It is easy
to connect a new switch fuse at any point of the busbars and the electrical installation is
thus both convenient and flexible.
In small workshops, for example, metalwork and engineering rooms in secondary
schools, the machines used may be small enough to make it practicable to serve a number
of them from one ring circuit. Each machine is connected to the ring through a fused
isolator or through a switch fuse. The fuse is necessary to protect the final connection to
the machine, which is necessarily of a lower rating than the ring main, and to protect the
internal wiring of the machine that will also be of smaller cable than the ring main. The
cables of the ring main should be capable of carrying at least 70 per cent of the total
current taken by all the machines, and it will be found that this very soon restricts the size
of workshop that can be treated in this way.
It should be appreciated that everything that has been said about power circuits applies
equally to three-phase and single-phase circuits. Where three-phase machines are used
three or four cables, according to the system, plus an earth connection, are installed, and
distribution boards, isolators and circuit breakers are of the three phase-pattern, but the
general circuit arrangements are the same as for single-phase circuits.
All mechanical equipment requires maintenance, and all machines and equipment
must, therefore, be installed in such a way that maintenance is possible. One of the things
that has to be done before maintenance work is started is the turning off of the electricity
supply, and it must be possible to isolate each machine or group of machines. It has been
known to happen that an electrician has turned off an isolator in a switch room and gone
to work on a machine some way from that room, that someone else has come along later,
Power 129
not realized that anyone was working on the machine and has turned the isolator on
again. Not only has this happened, it has caused deaths. Consequently, most safety
regulations, especially The Electricity At Work Regulations, now require that there should
be an isolator within reach of the machine, or is lockable–in any case the isolation must
be secure. The intention is that no one can attempt to turn the supply on without the
person on the machine becoming aware of what is happening. For small machines, such
as roof extractor fans, connecting the machine to the wiring through a socket and plug
near the machine is a convenient and satisfactory way of providing local isolation. For
larger machines, a switch or isolator or disconnector as it is now known has to be
installed.
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