How to test earth leakage causing RCD to trip

[abwooli]

Hope you all good guys

I have a quick question regarding how to test Earth leakage ie on socket, Boilr, 240v Aplaince or on RCD

And what the permitted range

What equipment to use
Can I use a normal multimeter?
Steps to take

Hope this question hasn't been asked before so am sorry to repeat. I have read on line watch few vidoes some say you need a earth leakage clamp meter, multimeter and soon on

Can I say am studying for my level 2 electrics but not all is covered so reaching and reading helps a lot

I am aware of the safety aspects of things

Thank you all

 

[ericmark]

Proper testing of a RCD needs special meters, you can't measure 40 mS with a stop watch. To test leakage you should use 500 volt, although where there is sensitive equipment you can use 250 volt, so to test correctly you need a special insulation tester.

However there are some tests which can be done with less expensive equipment.
1) The EZ150 plug in tester does measure loop impedance.
2) Some clamp on multi meters will measure 30 mA.
3) The built in test button on a RCD is in theory to test the mechanical parts, but it will test it trips with whatever size the built in test resistor is, it may allow much more than 30 mA to flow however.
4) Some simple plug in testers do have a resistor built in to test the RCD, however they do not measure the time.
5) With the EZ150 there is a current from line to earth, I seem to remember around 10 mA so it should not trip the RCD, however if the circuit is already on the edge, then this may trip it, so being able to use it does show it's not on the edge of tripping.

Even if the multi meter has a MΩ range it will not show insulation faults, the voltage is not high enough.

The normal house should have two RCD's, the easy way to test any item is to plug it into a circuit supplied from the other RCD and see if the other RCD starts to trip. Even with a insulation tester (they are also built into PAT testers) you may not find the fault when it's dependent on time or temperature, for example a de-frost element on a freezer likely only switches on once or twice a day, and once switched off, likely the timer is back to zero, you may find it if single pole switched testing the neutral to earth, but not always, washing machines again pump and spin dry clutches may only activate after some time.

 

[PrenticeBoyofDerry]

A clamp meter on the CPC, could be used to detect earth leakage. You could also clamp line under load and neutral under load and subtract the difference. Problem is, if you have a trip leakage current, the RCDs (if present) will trip once the fault range is reached.
Generally in a domestic installation the RCDs have a printed on value of 30mA, so once that is exceeded through leakage, the device will trip.
Yet most 30mA devices trip at a lesser value, which will vary between manufactures, but expect it to be nearer 26mA.
You can do a ramp test on the device to establish at what point it will trip, ideally this is done without any load connected, as that my subtract from the measured value.

 

[JohnW2]

A clamp meter on the CPC, could be used to detect earth leakage. You could also clamp line under load and neutral under load and subtract the difference....

... or, more simply (than the latter) just put the clamp around both line and neutral conductors (but not also the CPC).

However, as we recently discovered, at least some of the cheap clamp meters which theoretically have adequate resolution for the purpose have such poor accuracy at low currents as to be totally useless. The specification of the one we recently looked at suggested that any current less than about 30 mA could display as zero, whilst a displayed current of 30 mA could result from an actual current anything from zero to about 100 mA.

Kind Regards, John

 

[JohnD]

A clamp meter on the CPC, could be used to detect earth leakage.

I won't (all) be going down the CPC, depending on the appliance, but certainly not a boiler.

 

[abwooli]

Thank you to you all for the input am at work at the moment so havnt read fully in your replies
But
I have a megger mft1731 multifunctional tester will this do the job/ earth leakage

Or do I have to buy another tester to.

I am just learning how to use the megger the moment thanks lads

 

[EFLImpudence]

I have a megger mft1731 multifunctional tester will this do the job/ earth leakage

Not really. You can work it out by lots of measurement.

1. Disconnect the load side cables from RCD. Do ramp test. Say 26mA.
2. Reconnect cables. All appliances off. Do ramp test. Say still 26mA. No leakage in system.
3. Switch on an appliance. Do ramp test. Say now 24mA. Therefore 2mA leakage in appliance.

Or back to step 2.
2. Reconnect cables. All MCBs on and appliances off. Do ramp test. Say now 20mA. So 6mA leakage in system.
3. Disconnect L & N of suspected circuit. MCB off. Do ramp test. Say now 26mA again. So the 6mA must be on that circuit.
Repeat with MCB and appliance on with If still 20mA then leakage must be on another circuit.
4. Repeat from step 2 with other circuits.

There could be L to earth leakage which will confuse the readings.
There could always be some unwanted interconnection between circuits which will mess up the readings.

I may not have covered every option but you can work it out.

Or do I have to buy another tester to.

Yes, buy a good Earth Leakage Clamp Meter and test L & N together on Main tails - whole system, RCD tails - that part of system, or circuit conductors - just that circuit.

 

[JohnW2]

I won't (all) be going down the CPC, depending on the appliance, but certainly not a boiler.

Indeed, which is why a clamp meter (if it's up to the job - see above comments) around L+N (but not CPC) would be a better way to go.

PBoD's suggestion of separate L and N measurements, and then a subtraction, would be an alternative but (a) that seems like unnecessary work! and (b) if the L and N currents are high, and the meter auto-ranging (or, if manual, is switched to a higher current range), it might not have adequate resolution for the purpose.

Kind Regards, John

 

[abwooli]

Thanks guys for all your help