rcd tripping fixed

[chrisreid186]

here's one to be aware of

today i installed some lights, fan and mirror light in a brand new bathroom, all new cable etc, tested R1+ R2 and insulation resistance all good, the feed came from existing light circuit and went into my rcd, (power breaker unit)

so went to liven it all up and the rcd tripped straight away, so thought maybe i got a switch and feed mixed up after testing, belled all switch's out and they were fine, pulled all the lights down to make sure polarity was correct which it was

really couldn't work out why the rcd wouldn't work, then i decided to do a zs test from the supply side of the rcd which pointed out their was no earth, had a look at some of the existing lighting circuit joint box's and the cpc's had just been chopped off and left unconnected..

ended up running a fly lead back to a joint box which did have a decent earth and this corrected the fault. took a couple of hours to rectify this problem and from now on i will always check zs before extending a circuit rather then later ,

i think the problem was down to induced voltage (eddy current) on cpc's which were not correctly earthed causing the rcd to trip???

the earthing system was tns

 

[malcolmsanford]

I know it is the weekend, and I'm in holiday mode, but why would a lack of a CPC trip an RCD..................in fact if you had a fault by disconnecting the CPC often will keep an RCD on ...................have i missed something lads, help an old lad out here.

 

[chrisreid186]

that's what I'm trying to find out, i think its something to do with induced voltage? as soon as i connected my fly lead to a cpc that went back to the met the rcd was fine?

 

[chrisreid186]

i've seen something similar before where the main earthing conductor was disconnected and the cu energised (very dangerous i know) , you could measure around 70 volts between earth and live, again when the earthing conductor was connected this weird voltage went away,

 

[Deleted member 26818]

regulation 701.411.3.3 Additional Protection by RCD's

Additional protection shall be provided for all circuits of the location, by use of one or more Rcd's having the characteristics specified in regulation 415.1.1..........

where does it say the full circuit needs to be RCD...??? the OP has RCD'd the bathroom via a RCD FCU at origin of the power into the bathroom (in this case a feed from a lighting circuit) how has this not complied...!

Of course it doesnt state full, why would it. The Regs don't state full MCB or the full conductor when refering to those items.
The origin of the power into the bathroom is only part of the circuit, the Regs require the circuit to be protected, not some of or just a little bit of the circuit. If that was the case, then they would state something such as 'those parts of the circuits which enter the location'.
As stated before, the reason for this, is because any earth fault on the non-RCD protected part of the circuit, will allow earth fault current to be imported into the location.

 

[Des 56]

If an existing circuit of a location containing a bath or shower is extended, at least the extended part of the existing circuit must be provided with RCD protection. Supplementary bonding in a bathroom or shower room must be provided unless all the requirements in the 17th Edition for the omission of supplementary bonding are met.

Regulation number(s) 701.411.3.3 701.415.2


ESC

 

[Deleted member 26818]

Very strange there are 3 new regulations in the 17th regarding RCD protection which many electricians are unable to get to grips with.
The regulation requiring socket-outlets intended for general use by ordinary persons, requires the socket-outlets to be protected, yet invariably people believe that the whole of the circuit requires protection.
With the Regulation that refers to cables concealed in walls, again the requirement is only to protect those cables that are concealled in walls, but people will insist that the whole circuit be protected.
This Regulation which refers to locations containing baths or showers, is the only one that specifically refers to the circuit being protected, yet people believe that they can get away with only protecting part of the circuit.
If you do not protect the whole circuit, you haven't complied with the Regulations, and you might as well have not bothered installing an RCD at all.

 

[malcolmsanford]

I agree Spin with the sockets, there is nothing to stop you runing say a radial circuit in surface mounted trunking that as 4 double sockets and if those 4 sockets are BS 7288 RCD trype then you conform to regulation 411.3.3, it is just cost prohibitive and easier to comply with that regulation with a single RCD at the origin of the circuit.

Again with the cables buried in the wall, I suppose in theory you could run a cable in trunking surface to a double pole switch for say a washing machine, and then from the switch you bury the cable to a single undercounter terminal plate, in theory I suppose you could fit a RCD in just that part that runs buried but again it is easier and safer to protect the entire circuit with a RCD at origin.

As for the circuit to a bathroom, hmmmm I think that convention in the industry favours the RCD on just the part of the circuit that enters the location, but I can not if you read the reg say that your wrong. It may well indeed be that your interpretation is right, I would not like to argue with that.

 

[Deleted member 9648]

Of course it doesnt state full, why would it. The Regs don't state full MCB or the full conductor when refering to those items.
The origin of the power into the bathroom is only part of the circuit, the Regs require the circuit to be protected, not some of or just a little bit of the circuit. If that was the case, then they would state something such as 'those parts of the circuits which enter the location'.
As stated before, the reason for this, is because any earth fault on the non-RCD protected part of the circuit, will allow earth fault current to be imported into the location.

Bumping this up again because I initially queried spins posts on this and I've not had my BRB...or forum access till this morning.
This is an important point as far as I'm concerned because if spins interpretation is correct an awful lot of people are not complying with the regs.

Spin IMO and with respect your interpretation is incorrect.
A fault current from a non RCD protected part of the circuits feeding the location cannot be introduced into that location....a current will flow from the fault down the CPC and to earth via the path of least resistance,as far as the shock risk is concerned the current flowing as a result of the fault is irrelevant,it is the voltage which results in a shock risk.....For the duration of that fault a voltage will appear on all conductive parts of the install,including the locations.
Regardless of the type of protective device at the origin of the circuit an earth fault will introduce a voltage to all conductive parts of the installation until the protective device clears it,because all conductive parts are connected together. An RCD at the origin of the circuit may limit the duration of the fault voltage on conductive parts,but bs7671 does not require different disconnection times for bathrooms and shower rooms.
If your argument applies then you will have to provide RCD protection to the origin of all circuits in the entire install,not just the location.....as an earth fault on any circuit will introduce the fault into the location ,as all conductive parts are connected together.

For those reasons I believe Spins take on this is not correct and 701.411.3.3 is not requiring the entire circuit to be RCD protected.

 

[Deleted member 26818]

Whilst I agree that an earth fault on any other circuit will cause a rise across all circuits, and on all connected conductive parts, thereby bypassing any RCDs at the origins of the circuits.
However, what would happen if there is a break in the CPC on the circuit between the origin and the RCD?
What will then be the fault path?
You disagree with me on my interperetation, which is your perogative.
However you have not explained why this particular Regulation, and the one further on in the chapter under Supplementary bonding, refer to circuits and final circuits being RCD protected, if the requirement is not to protect the circuits.

 

[Dazza1976]

I know it is the weekend, and I'm in holiday mode, but why would a lack of a CPC trip an RCD..................in fact if you had a fault by disconnecting the CPC often will keep an RCD on ...................have i missed something lads, help an old lad out here.

I was going to ask this same question, The RCD detects an inbalance in the phase and neautral, so i too am failing to see why it would trip with the cpc missing ? If anything you think it would be less likely to trip without one ?

 

[chrisreid186]

i think its either that some rcds require a functional earth and won't operate without one? or something to do with induced voltage? but as you said rcd's only monitor the live and neutral conductors so wouldn't matter if the earth had a small voltage on it?