I've just had a new split load CU fitted. Never had RCD protection before. When the spark (Part P regd) connected the oven circuit, it tripped the RCD. This is a single 2.5mm cable from the CU to an under oven (not hob), not part of any ring etc. I know, I installed it myself about 15 years ago, and it's never caused any problems. He isolating the oven - i.e. disconnected it from fused spur outlet and left cable ends bare, so no appliance induced issues. But it still tripped RCD. Unfortunately cable is now under unnaccessable floor and in kitchen wall. He asked if there's ever been any problems. I said no, and there's been no building work done in area - i.e screw through cable etc. So he decided to connect to just MCB circuit. Which doesn't trip. He said this would be alright. Obviously there's some current leakage somewhere. My uneducated guess could be rodent damage under the floor exposing the cable and maybe just a little bit of damp. But what do you think?
RCD trips - MCB doesn't
- Thread starter jonmoz
- Start date
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It is quite usual to have cookers unprotected by RCDs as they tend to get a bit of leakage with age, so that's not wrong.
However, in your case, you appear to have untraced earth leakage on the cable. As you say, it might be due to rodent damage. It would be useful to know how bad it is and whether the faulty part of cable should be replaced.
Did your electrician carry out an insulation test?
Replacement of a Consumer Unit has to be notified to your local authority Building Control office - was this done? Did the electrician test the circuits and provide you with an Installation Certificate? You say the electrician was "Part P regd" which I understand to mean that if he is a member of one of the industry schemes for inspection and testing. Which one, and what documentation did he leave with you?
However, in your case, you appear to have untraced earth leakage on the cable. As you say, it might be due to rodent damage. It would be useful to know how bad it is and whether the faulty part of cable should be replaced.
Did your electrician carry out an insulation test?
Replacement of a Consumer Unit has to be notified to your local authority Building Control office - was this done? Did the electrician test the circuits and provide you with an Installation Certificate? You say the electrician was "Part P regd" which I understand to mean that if he is a member of one of the industry schemes for inspection and testing. Which one, and what documentation did he leave with you?
He said he'd done an insulation test, and it had "passed".
Got all the doc bumf, and notification duly done, etc.
I'm happy with all the above, just a bit concerned about the tripping / non tripping. Does this mean that I'm wasting electricity? Or is there a potential fire hazard?
Got all the doc bumf, and notification duly done, etc.
I'm happy with all the above, just a bit concerned about the tripping / non tripping. Does this mean that I'm wasting electricity? Or is there a potential fire hazard?
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JohnD said:
But evidentally the leakage was large enough to trip a 30mA RCD. Personally. i'd not be satisfied.
Interesting idea of "passed".jonmoz said:
According to my sums, to get a 30mA leak, you need a P/E or N/E resistance of less than 7667 ohms...
So what? for a piece of cable ohms is ohms.
For it to trip the RCD (per above) it must have an earth - phase or neutral fault of 7.5K ohms or less and thats a fail on the insulation test.
TTC
For it to trip the RCD (per above) it must have an earth - phase or neutral fault of 7.5K ohms or less and thats a fail on the insulation test.
TTC
Just to clarify, an insulation resistance of less than 0.5M ohms @ 500v is a fail and anything under 2M ohm should be investigated further.Taylortwocities said:
On the other side of the coin, could it have a borrowed neutral?
Will an earth to neutral fault of 7.5K ohms resistance trip an RCD, where the PD between N&E is minimal, often less than 5v?
Not according to Mr Ohm.
If he's right, the resistance needs to be less than 167.
If he's right, the resistance needs to be less than 167.
All this technical stuff may be very interesting to specialists, but it doesn't help me; and it doesn't answer my original question. Is this acceptable / safe? You sound like it's a game to come up with the most complex piece of technospeak. As a reminder, take a look at the name of this website
Here I am - a "customer", have I been ripped off by another dodgy electrician? It took me ages to get someone to do it, in the first place. Maybe the job was too small, I don't know. 20 years ago, I'd have got stuck in and sorted it myself, but at 60+ (and recent regs) it's not worth it. Before adding the new CU and RCD protection, everything seemed fine. The cable to the oven, was obviously intact when installed and was wired via a 15A DP fused switch. Never caused any problems. Still not causing any problems, as such. MCB happy. But RCD not.
Here I am - a "customer", have I been ripped off by another dodgy electrician? It took me ages to get someone to do it, in the first place. Maybe the job was too small, I don't know. 20 years ago, I'd have got stuck in and sorted it myself, but at 60+ (and recent regs) it's not worth it. Before adding the new CU and RCD protection, everything seemed fine. The cable to the oven, was obviously intact when installed and was wired via a 15A DP fused switch. Never caused any problems. Still not causing any problems, as such. MCB happy. But RCD not.
I stand by my reply Posted: Thu Sep 28, 2006 10:23 am
All the talk that's gone on has been trying to guess what might have happened, by people who haven't looked at your job.
But if you can find the test readings for the cooker circuit on your documentation, put them up here and we might be able to do some remote diagnosis.
My opinion is that's it's a pity the cooker cable is in an inaccessible position; otherwise it might have been quickest to run a new piece and job would be done. I imagine the connections in the cooker switch and the CU were checked by your electrician so that only leaves the cable (and perhaps your rodents).
All the talk that's gone on has been trying to guess what might have happened, by people who haven't looked at your job.
But if you can find the test readings for the cooker circuit on your documentation, put them up here and we might be able to do some remote diagnosis.
My opinion is that's it's a pity the cooker cable is in an inaccessible position; otherwise it might have been quickest to run a new piece and job would be done. I imagine the connections in the cooker switch and the CU were checked by your electrician so that only leaves the cable (and perhaps your rodents).
JohnD
You're the only person on here, who talks sense. I'll get documentation out and post next week. The oven was totally disconnected from the supply cable - I saw bare cable ends at both CU and fused outlet - so the problem has to be in the cable itself. I'm still on the mouse/rat theory, because I've had other (non cable) damage over the years under the floor. I installed the cable (as part of a total house rewire) circa 1990, so I know the cable runs from "under the stairs" along the floor of the foundations (concrete), before rising up in a wall (sand & cement rendered). Other than a beast, there's no way that the cable could have been damaged in any way or form.
Am I right in assuming that a live - neutral short would trip the MCB and obviously the previous fuse, but that a neutral - earth short wouldn't cause a trip. But would trip an RCD?
You're the only person on here, who talks sense. I'll get documentation out and post next week. The oven was totally disconnected from the supply cable - I saw bare cable ends at both CU and fused outlet - so the problem has to be in the cable itself. I'm still on the mouse/rat theory, because I've had other (non cable) damage over the years under the floor. I installed the cable (as part of a total house rewire) circa 1990, so I know the cable runs from "under the stairs" along the floor of the foundations (concrete), before rising up in a wall (sand & cement rendered). Other than a beast, there's no way that the cable could have been damaged in any way or form.
Am I right in assuming that a live - neutral short would trip the MCB and obviously the previous fuse, but that a neutral - earth short wouldn't cause a trip. But would trip an RCD?
jonmoz said:
jonmoz said:
A dead short would, but reduced resistance wouldn't (unless extremely bad - and your electrician would have spotted that, I'm sure).
jonmoz said:
A neutral/earth fault wouldn't have any effect on a fuse or an MCB. But it could trip an RCD, depending how bad it was, especially if there was current draw on the affected circuit.
Additionally, even a small Phase/earth fault would almost certainly trip the RCD, as 30mA is a very slight current.
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