ring circuit headroom reduced

Lanca · Posted: Wed Jul 05, 2006 5:49 pm Post Subject: ring circuit headroom reduced

I have just tried to extend an existing ring circuit to add a new socket, but have run into problems. I located the ring cable, cut it and installed new cable to the new outlet and back again using 30A junction boxes. However as soon as i tried to run something from the new outlet the RCD at the consumer unit tripped. Assuming I had introduced a fault somewhere I then disconnected the new cable and connected the two junction boxes to restore the original loop, minus my new addition. However, although the circuit functions it still trips the RCD very easily if anything of reasonable power (eg. vacuum cleaner) is started. Before my meddling this circuit was very stable, never had any problems. It is not overloaded, only a PC and TV most of the time. Its as if the headroom has been reduced significantly, but I don't understand how that can be. If there's an earth fault or something on the circuit wouldn't the RCD refuse to re-set ? Grateful if someone could point me in the right direction on how to troubleshoot this.

JohnD · Posted: Wed Jul 05, 2006 5:58 pm Post Subject:

Perhaps ypu have mis-connected the Neutral (black, or blue) and the Earth (green, green-and-yellow, or bare copper) together or touching each other, including perhaps in one of the steel back-boxes.

Have you sheathed all the earth wires?

Did you touch anything at the consumer unit?

Lanca · Posted: Wed Jul 05, 2006 6:06 pm Post Subject:

Thanks for the prompt reply. I haven't touched any of the steel back boxes, all of the work was done in the loft. I have checked the wiring, brown to red and blue to black all seems ok. Can't see any nicks in the sheathing that might short, and yes I have sheathed the earths. Didn't touch anything at the consumer unit other than to re-set the RCD.

plugwash · Posted: Wed Jul 05, 2006 8:17 pm Post Subject:

assuming all the new connections are correct and well made (no little metal tails etc) im guessing that your movement of the cable as you cut and joined it caused the fault.

you need to trace the cables from your new junction boxes to the existing sockets and look for any damage.

do the cables in question run down metal conduit in the wall? if so are the conduit ends digging into the cable (very common on diy rewires where the bulk of the conduit was ripped out but conduit in the walls left and T&E pulled through it)? entry points to sockets are also possiblly vulnerable.

bondy · Posted: Wed Jul 05, 2006 8:31 pm Post Subject:

have you checked the rcd?

Lectrician · Posted: Wed Jul 05, 2006 9:18 pm Post Subject:

What else is protected by the RCD?

Lanca · Posted: Wed Jul 05, 2006 10:10 pm Post Subject:

Thanks everyone for the help. I have disconnected everything once again and re-made the connections and everything seems ok at the moment. Hopefully it was just a dodgy connection somewhere, or I may have moved the cables fortuitously this time. I am still curious though why the RCD only tripped after a certain load was exceeded, shouldn't it trip automatically if there is a fault ?

Adam_151 · Posted: Wed Jul 05, 2006 10:43 pm Post Subject:

Lanca wrote:

I am still curious though why the RCD only tripped after a certain load was exceeded, shouldn't it trip automatically if there is a fault ?

If there is a neutral->earth short, the additional load will drag the neutral away from earth voltage, creating a potential between neutral and earth which shunts a current through the fault and trips it, quite a few varaibles in it

The resistance of the fault
The neutral-> earth voltage normally (this willl vary throughout the day as loadings change)
How much more the neutral moves away from earth with the load you have applied (current of the load, and resistance of the conductors feeding it relevant here)
Etc, and because multiple circuits might be loaded and the fault is only one one, it becomes rather complicated if you wanted to calculate the leakage generated by this effect