Ring mains in the Home
Many houses have electrical circuits which are inadequate for present-day demands. Many older homes were designed when electrical appliances that are in wide use today were not available.
If your poweris overloaded and inadequate, rather than trying to modify it, it is better to rip the whole lot out and install a ring main, now the standard method of wiring domestic power circuits.
A ring main is a circuit which starts and finishes at the consumer unit. Flat three-pin 13A plugs and sockets are used.
Before installation, the siting of sockets should be planned. First, you can have a ring circuit supplying up to 100m2 of floor area. This gives a combinedof 7.2kW.
If the floor area of your home is, say, 170 m2 you will need at least two ring circuits.
Each circuit will need its own 30A fuse, so a new consumer unit will have to be fitted, preferably on a new board.
When siting two ring-main circuits, the loading should be equally distributed on both. While you can have an unlimited number of sockets on a ring main, what is important is the loading of appliances actually plugged in at one time.
Wattage can be calculated by multiplying voltage by current. Since AC mains are about 240V, if an appliance draws 10A, then its wattage is 240 x 10 = 2400 watts or 2.4kW.
Conversely, if an appliance is rated at 2.4kW, you know that it will draw 10A, and, therefore, must have a 13A fuse to protect it.
Using this formula you can calculate what loading will be expected on a circuit, and the fuse that should be used to protect the appliance. Lamps drawing about 150W need a 3A fuse. TV sets, and, particularly, colour receivers, should be fitted with 13A fuses.
Toasters, irons and single bar fires draw about lkW and need a 13A fuse; three-bar heaters and high-speed kettles draw about 3kW and need 13A fuses.
When planning a ring main, do not skimp on the number of sockets you install, it is easier to put them in when you do the job, and parts are not costly.
Sometimes it will be inconvenient to run a ring circuit to some points, for example, a socket in thefor a lead lamp. For these applications, a fused spur can be used.
Each spur can have up to two sockets on it and two separate sockets or a double one can be used.
The number of spur sockets must not exceed the number of sockets on a ring circuit.
Some items, like waste eliminators, are permanently fixed into position, and, therefore, do not need a plug and a socket.
In this case, a fused spur is used to connect them into the ring main. The fuse rating can be calculated from the formula. Only one appliance may be connected to one spur.
An electric clock is a low-current device and is wired in using a special 1A fused clock connector.
Electric cookers and immersion heaters are not wired into the ring main if they exceed 3kW. The maximum load that can be connected to a fused spur.
A cooker is usually about 8kW and, thus, is wired independently to a 45A fuse at the consumer unit. The appliance is also wired through a control switch outlet situated near the appliance. Heat-resistant cable should be used when connecting cookers or immersion heaters to their outlets.
The cable used for wiring a ring main must be of a rating suitable for the loading to be imposed on it, otherwise it may overheat. The type used is 2.5mm2 twin-conductor-with-earth insulated-and-sheathed cable.
Some types have a red live wire, a black neutral wire and a green or unsheathed earth wire.
New cable, consisting of a live, sheathed wire and two unsheathed wires, the negative and the earth, called combined negative earth. (CNE), is coming into use. The unsheathed wires are bonded (joined) at switch and appliance. Wires are sheathed in PVC.
Cable supplying the consumer unit from the meter should be 16mm2.
You may mount a wall radiant heater in a bathroom as long as it is out of reach and is operated by a pull cord switch. No portable electrical appliance should ever be used in a bathroom.
Socket outlets should be placed to avoid trailing flexes and use of multiple adaptors. A suggested minimum number of positions is: Kitchen, four: living area, three: dining area, two each; bedroom, two: hall and landing, garage (where integrated with the house), store or workroom, one each. Use switched outlet sockets with indicator neons for preference, particularly where there are children.
A radial circuit is a single-cable, installation. Independent of the ring main. Taken from the consumer unit or through a fuse.
This may serve one room of less than 30m- in area, and supply up to six single or three double 13A outlets. It may not be used in a kitchen, which may contain high-wattage appliances.
The circuit, in twin 2.5mm2 cable, with earth, must be protected by a 20A fuse or MCB. Is limited to two single, one double. Or to two fused spurs for fixed appliances.
With a room of more than 30m2 in area. 4mm2 cable should be used, protected by a 30A fuse or MCB. Up to six 13A outlets may be used for fixed appliances.
All installation wiring is the same as described for lighting. Surface wiring and underfloor cabling follow the same rules, and earthed metal wall boxes must be used to mount the sockets.
Sockets should be fitted at least 180mm above the floor.
10. November 2011 by admin
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