Intermittent RCD Problem ???

[JimG]

Hi Folks,

Have an intermittent RCD tripping problem on kitchen/upstairs ring ....... MCB not tripping.

On-site today checking-out this issue. Removed all known appliances from ring - RCD still tripping ?

Removed a socket - checked I/R between L/E - meter indicating short.

Removed numerous additional FCU's in an attempt to isolate the possible short ??

Decided to re-set-on the RCD and it holds ???

Can someone advise me why an RCD should hold-on when my meter is telling me I have 0.00 I/R ??

 

[Evans Electric]

Funny things , RCDs ,sometimes . Need to test RCD. If everything is unplugged you're looking at dividing the ring up until it clears , looking for damp or nail/screw in cable or something dodgey still connected , I'm sure you know all this but there you go.

 

[JimG]

Thanks Deke - but can you tell me why my meter tells me there's a short between L/E on the ring - yet the RCD holds-in ????????

 

[ProDave]

Because it's not a SHORT, just a low number of megohms.

Try it on the earth continuity range and it will probably give you a high reading.

In order to cause 30mA of leakage, and hence trip the RCD would require a resistance of 6800 ohms or less. So say a leakage of 10,000 ohms would not trip an RCD, but would still read virtually zero on an insulation tester.

The other thing to look for is rodents, dead or allive. Last week I found a tripping RCD was due to a dead mouse slumped over the MCB inside the CU.

 

[Evans Electric]

I'm wondering if the RCD is faulty and is not tripping immediatly , when it should , hence the test . As you say ,the MCB does'nt trip so it can't be a dead short .

As you can tell , I'm not sure on this , a damp reading can cause this sort of thing although a 500V megger test shows a dead short . Back in the stone age when we used analogue meggers we would get the needle oscilating up and down the scale and often meant there was damp in, say, a pyro or an underground joint .

 

[choccy]

hiya jimg i had the same problem last week rcd tripping when load put through but holding when nothing plugged in. tested ring and indicated a short between live and earth so split ring down as you do and got to one socket where there was a tiny nick in the neutral cable and it was touching the earthed back box.

repaired cable and everything clear and ok.

so i think you need to remove everything from the circuit and start testing. :|

 

[Doc Hudson]

Removed a socket - checked I/R between L/E - meter indicating short.

0.0Mohms is nowhere near a short, remember 1.0Mohms is 1,000,000ohms, so a fault of 99,999ohms would show as 0.0Mohms. A low resistance continuity tester is needed to establish if there is a short.

Doc H

 

[Larnacaman]

If your absolutely sure that there are no other appliances left connected on the Ring, then check out for any evidence of a previously trapped wire from the plate screws. If there is a particually bad trapping, but hasn't quite penetrated the insulation completly then it is possible to get a 0.00 IR reading, with a 500V megger.

Disregarding the IR reading, the other thing to check is the total leakage current shown to the RCD (is it an rcd or rcbo?) when everything is working in normal situation, or check the device with a ramp test. Remember 30mA RCDs can trip at a lower value than the stated 30mA... There is a tolarance value on all stated RCD values.

If all tests prove satisfactory, and this RCD is still tripping intermittently then your going to have to check each ring section out, ...outlet to outlet!!! Grrrrrr!! lol!!!

 

[JimG]

Hi All - Thx v-much for yr thoughts/inputs.

After much searching & measuring, I eventually got there Unbeknown to the folks of the house, previous owners had installed some kinda water softener gizmo right in the corner of the kitchen -- yes -- behind the kitchen corner unit, being supplied from an old extension lead which was plugged into a hidden d/skt behind yet another low-level unit. I noticed the cable to the ext-lead after removing the 3rd kicker panel/plinth.

Chked the water sotner gizmo and it was S/C between L & E --- probably rusted to hell.

Diss'd the ext-lead & gizmo - all is back to normal

Chk'd RCD - 1n @ 25mS // 5n @ 12mS.

Can anyone supply X-Ray goggles ??