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RCD and Minimum Insulation Readings


chris_k

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Evening all.

Just a quick one. May be a silly question but, how low can an insulation reading be before an rcd will trip?

Cheers

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Andy™

for 30mA, 7666.6 ohms. but in reality, it will differ

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Revved Up Sparky
for 30mA, 7666.6 ohms. but in reality, it will differ

Yes, R = V/I = 230/0.03 = 7666.6 ohms

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Chris is this a purely technical question or is it related to an actual installation. are you aware of minimum Insulation Resistance values in the regs ?

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septiclecky
Evening all.

Just a quick one. May be a silly question but, how low can an insulation reading be before an rcd will trip?

Cheers

An RCD should not trip if you are doing an insulation reading as there is no power on the circuit hopefully:|

A 30mA RCD will trip only if there is a imbalance of more than 30mA between phase and neutral nothing to do with the insulation resistance of the circuit.

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Chris is this a purely technical question or is it related to an actual installation. are you aware of minimum Insulation Resistance values in the regs ?

Yeah i know minimum IR for circuits. Ill expand more when i get time tonight.

Chris

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Revved Up Sparky

....A 30mA RCD will trip only if there is a imbalance of more than 30mA between phase and neutral nothing to do with the insulation resistance of the circuit.

That imbalance is caused by some current (ie, >30ma) leaking down to earth instead of it returning through it's correct return path (L-N). Insulation failure between L/N &E will create a low resistance between L/N & E, which if low enough, will allow 30ma to drain to earth and thus activate the RCD. Typically a switchwire trapped against a metal box by a pattress screw will cause such a low resistance to occur. Therefore my point is that earth leakage causing an RCD to trip can indeed occur as a result of low insulation resistance.

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I most definately agree with you on that point R.U.S, and if the circuit connected to this RCD also have additional earth leakage from connected loads etc then then any low values of cable insulation resistance become even more of a potential trip problem.

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....how low can an insulation reading be before an rcd will trip?

Well here goes.

I had a job sprung on me last minute, bungalow conversion to dormer. Involving widening and raising the roof. On my initial inspection all was ok ish. TNCS , Other services next to cu, gonna need new board for rcd protection,

Plan

2 extra circuits to add to board for upstairs, Change a few jbs for non maintenance ones. easy job.

Well when builder starts job i get a call "..can you just come and have a look?"

I arrived to find all insulation removed and a mass of wires. Like a spiders web. Wires going everywhere. Counted 24 JBs, Rubber cable, old colours, new colours, Steel wire, JB`s with no covers, Choc blocs everywhere untaped. Decide to crack open cu. MY GOD. Rats nest of above wires. Best was 2.5mm into incoming side of main switch to feed a COOKER because not enough mcb`s.

Turns out the homeowner used to be an electrician down the pit. :_|

Cant have builders working around live stuff so crack on with making safe. found ringmains in multiple loops all interconnected, same with lighting circuits. Ended up removing 50% of the wiring, Connected up all dropped earths.....Tell builder and householder will have to test the lot and see what happens.

Testing arrives. Faults were everywhere. One lighting circuit had a direct connection between live

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Chris that looks to me like 11.5 mA leakage.which on its own will not trip a 30mA rcd, but you need to consider what additional leakage there is on the existing installation....by the sound of the condition of this property there is deffo going to be..What is the overall I.R L/N to Earth?

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boltonsparky
says it was alright before we turned up

Heard that too many times before now!

It hasn't killed me yet so it must be ok attitude headbang

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Revved Up Sparky

Well when builder starts job i get a call "..can you just come and have a look?"

I arrived to find all insulation removed and a mass of wires. Like a spiders web. Wires going everywhere. Counted 24 JBs, Rubber cable, old colours, new colours, Steel wire, JB`s with no covers, Choc blocs everywhere untaped. Decide to crack open cu. MY GOD. Rats nest of above wires. Best was 2.5mm into incoming side of main switch to feed a COOKER because not enough mcb`s.

Turns out the homeowner used to be an electrician down the pit. :_|

Chris, at this stage I would have contacted the owner and informed him that the property needs a full rewire immediately. If he refused I would have walked away.

I would disconnect that garage feed and leave it out of the circuit as IR should'nt be less that 1 megohm (tabe 10.1 OSG).

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  • 4 years later...
Lee Sparks

Yes, R = V/I = 230/0.03 = 7666.6 :ohms

Cool, but what would this be in Mega Ohms? 0.07ohms?

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Andy™

0.007

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ProDave

Thread resurected from 2009

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kerching

Is this a resurrection record?

Just.....joining in

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Doc Hudson

Cool, but what would this be in Mega Ohms? 0.07ohms?

 

 

0.007

 

Because 1 MegOhm is 1,000,000 ohms, as Andy said 7666.6ohms would be 0.007666 MegOhms. But if your meter only shows two decimal places on the MegOhms scale 7666.6 ohms would actually show as 0.00Megohms, or Zero Meg. Some 'electricians' get confused thinking that a zero reading when doing an insulation resistance test is a short circuit, but as it is on the MegOhms scale Zero meg results could still be over 9000ohms, which is no where near a short circuit..

 

Doc H.

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Andy™

Thread resurected from 2009

internet has just reached Surrey

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Doc Hudson

Thread resurrected from 2009

 

 

Is this a resurrection record?

Just.....joining in

 

It is indeed a long resurrection period. What is more surprising is that it is not just an spammer advertising their overseas electrical supply company, but a genuine question relating to the original topic. Which may make it an even more impressive record?

 

Doc H.

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Lee Sparks

Cheers chaps, interesting.

 

Was doing a search on forums for various things and this post got my attention!

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kerching

I was doing some intrrnet research the other night, something got my ATTENTION also

Just saying

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I think the biggest problem with a borderline IR test is that whilst it might be above the tripping threshold of the RCD at the time of testing the IR won't be stable so you're almost guaranteed future problems. To be honest assuming no appliances or loads plugged in I'd consider any circuit testing <10Megs @ 250v worth investigation.

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Steve3948

I think the biggest problem with a borderline IR test is that whilst it might be above the tripping threshold of the RCD at the time of testing the IR won't be stable so you're almost guaranteed future problems. To be honest assuming no appliances or loads plugged in I'd consider any circuit testing <10Megs @ 250v worth investigation.

With digital meters these days we have become paranoid with readings, with the old wind up if the handle turned easy it was clear, a quick look and over 1 meg, classed as clear and move on, it was acceptable to have 0.5 Meg on conduit install, so what is the difference between 1 Meg and 10 Meg, both are clear readings as far as required to comply. I would not waste any time trying to locate a reading of 10 Meg, but would try to find what was causing a reading close to acceptable on a new install.

 

If the meter had its last reading as 1 Meg and then infinity would we worry.

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