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swa cable for shed socket

Postby olivier » Fri Dec 11, 2009 11:54 am

I would like to run a swa cable 2.5mm2 from a 16A MCB on a 30mA RCD protected CU to my shed (5m away) and connect to a junction box in the shed to power a double socket, an inside light and an outdoor PIR light. I am planning to have everything to IP55 or more.
- Would I need to have the 2G socket RCD protected? or change the MCB for a RCBO?
- Is the swa cable ok as a radial circuit or would I need to have it as a ring circuit?
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Postby moggy1968 » Fri Dec 11, 2009 8:34 pm

sorry, bit confused. is the RCD on your main CU or on a sub board in the shed
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Postby ericmark » Sat Dec 12, 2009 10:32 am

314.1 Every installation shall be divided into circuits, as necessary, to (iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation.
531.2.4 An RCD shall be so selected and the electrical circuits so subdivided that any protective conductor current which may be expected to occur during normal operation of the connected load(s) will be unlikely to cause unnecessary tripping of the device.
531.2.9 Where, for compliance with the requirements of the regulations for fault protection or otherwise to prevent danger, two or more RCDs are in series, and where discrimination in their operation is necessary to prevent danger. the characteristics of the devices shall be such that the intended discrimination is achieved.
536.3 Selectivity between RCDs
Where selectivity between RCDs is necessary to prevent danger and where required for proper functioning of the installation, the manufacturer's instructions shall be taken into account.

The rules go on and on but if the shed would be likely to cause unwanted tripping of the RCD then you must do something to stop it tripping the rest of the house. Feeding the shed through a 10ma RCD would achieve this. The tripping value for a RCD is somewhere between half the rated value and the rated value so to stop tripping the next RCD should be 1/3 of the value of proceeding RCD. There is also time and with a large fault both RCD’s could still trip. If the second RCD was in shed where under fault conditions you may get a volt drop which could cause a passive type to fail to trip then you must use an active type.

From your description it is unlikely you need a second RCD but no one could with the information given be certain about that.

I would not use a junction box. I would use a switched fuse connection unit to control lights so I could fuse down to 5A and use the socket back box or FCU back box as junction box. Likely using tube nuts to join them.

2.5mm² on a 16A MCB can be a radial and if already using RCD protected supply no point in RCBO.

However this will all come under Part P and the LABC sometimes do have odd ideas and best way is to detail what you intend before applying so anything they don’t like they can tell you about before you start.

They are normally quite good and will give you some guidance. Well at £100+ they should. Of course you could opt for cheap option and not DIY. Registered electricians don’t pay as much to issue completion certificates so they are around £90 up on you to start with plus they can get materials cheaper so for less than half a days work DIY is expensive option.
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