Testing 100mA RCD trips?

[Chris Wilson]

Can I put a resistor of suitable value across live and earth of a  of a mains plug and pop it in a 13 Amp switched socket and turn the socket on to check if a 100mA RCD trips? If so what value resistor? I know this is somewhat unconventional! I want to be able to use a remote controlled mains adaptor with a suitable plug / resistor  in it to trip an RCD to kill house ring main power remotely. Thanks.

[langwadt]

Can I put a resistor of suitable value across live and neutral of a  of a mains plug and pop it in a 13 Amp switched socket and turn the socket on to check if a 100mA RCD trips? If so what value resistor? I know this is somewhat unconventional! I want to be able to use a remote controlled mains adaptor with a suitable plug / resistor  in it to trip an RCD to kill house ring main power remotely. Thanks.

a resistor from live to neutral is just a normal load, an RCD trips when the current in live and neutral isn't balanced i.e. there is a load to ground 

[viperidae]

To trip an rcd, doesn't the current need to flow via the earth connection, not the neutral?

An rcd measures the difference in current between the live and neutral

[Chris Wilson]

Sorry, was rushing and put neutral instead of earth. Of course live and neutral would just be a normal (or abnormal...) load. I have edited the original post.

[fubar.gr]

A purely electromechanical* RCD like those typically used in Europe doesn't care about voltage. All it knows is current.

So instead of testing it at mains voltage, you can use a 6 or 12 Volt transformer (the lower, the better) and a pot of the appropriate wattage in series to vary the current. You can then hook the multimeter in series to measure the AC current.

Connect the transformer on the RCD in such way that the current flows from Line in to line out. Don't connect anything on the neutral terminals.


* I suspect that those RCDs with electronic components sold in the US won't work well with smaller voltages

[grumpydoc]

Can I put a resistor of suitable value across live and earth of a  of a mains plug and pop it in a 13 Amp switched socket and turn the socket on to check if a 100mA RCD trips? If so what value resistor? I know this is somewhat unconventional! I want to be able to use a remote controlled mains adaptor with a suitable plug / resistor  in it to trip an RCD to kill house ring main power remotely. Thanks.

That seems a slightly odd thing to do - care to expand?

Also, aren't domestic RCDs usually 30mA?

[DenzilPenberthy]

Can I put a resistor of suitable value across live and earth of a  of a mains plug and pop it in a 13 Amp switched socket and turn the socket on to check if a 100mA RCD trips?

Yes, That's what the 'test' button on the RCD does. Use Ohm's Law to calculate your resistor.  Bear in mind that the power dissipation will be quite substantial (at least 23W) if the RCD does not operate or if you plug it into a non RCD protected circuit. It is worth using a generously rated power resistor even if it will only be loaded for a few 50Hz cycles.

I want to be able to use a remote controlled mains adaptor with a suitable plug / resistor  in it to trip an RCD to kill house ring main power remotely. Thanks.

RCDs to protect socket circuits should be 30mA. If you have a 100mA RCD it's probably fitted between your meter and consumer unit to protect the whole installation.

[DenzilPenberthy]

I want to be able to use a remote controlled mains adaptor with a suitable plug / resistor  in it to trip an RCD to kill house ring main power remotely. Thanks.

That seems a slightly odd thing to do - care to expand?

On some electricians' forums I frequent I've seen one person do this to his noisy downstairs neighbour - introducing a L-E fault to their lighting circuit (accessed via his bedroom floorboards) to turn off their power when they are partying at 3am. Also another person fitted a remote control device like this well hidden in a proper nightmare customer's house. If he happens to be passing in the evening he'll just pop their electric off and have a little chuckle to himself.

[tggzzz]

Can I put a resistor of suitable value across live and earth of a  of a mains plug and pop it in a 13 Amp switched socket and turn the socket on to check if a 100mA RCD trips?

Yes, That's what the 'test' button on the RCD does.

Apparently not necessarily: some devices just mechanically trip the relay. That might be sufficient to reduce the chance of stiction preventing a real fault from tripping the device, but it isn't sufficient for the OP's test.

[Jeroen3]

Use a parallel current transformer to inject current into 1 channel of the RCD. This way you can test them operational (under normal conditions) with controllable current.
You can also create a real leak to ground. But this might trip the RCD before the DUT.

[Delta]

Wire an incandescent lamp up to L&E and plug it in when needed.  No chance of resistors catching fire...

PS. 100mA RCDs are used as main incomers when the earth fault loop impedance is too high, 30mA ones are used to protect final circuits...

[Ian.M]

Obsolete ELCBs that sensed the current in the protective Earth conductor may have used a test button load between Live and Earth, but the test button on a RCD does *NOT* apply a load between Live and Earth.  Many RCDs don't even have an earth connection, and for those that do, it would be hazardous to have a test button that would make the protective Earth live if it had an O/C fault in the feed to the RCD.      Instead, it applies a load from Live on the downstream side to Neutral on the feed, which results in a current imbalance between Live and Neutral through the device, simulating the same fault current to ground.   

There's also the issue of repeated tripping.   If the RCD is carrying close to its rated current, it may well be degraded by tripping it and failure within as little as 50 trip events is entirely possible.  Any RCD that meets the standards required in developed countries should be designed to fail safe, so the usual consequence is it fails open.  A thermal RCBO that is repeatedly tripped is likely to become more sensitive to current due to contact damage increasing its self-heating.

As 100mA RCDs are usually used as the main supply disconnect, replacing them is a total PITA, as you will have to break the tamperproof seal on the electricity company's fuse to isolate the supply, and will need to get the seal replaced before the meter reader sees its missing.   Therefore tripping them deliberately more frequently than for an annual test is a really bad idea.

[tggzzz]

Wire an incandescent lamp up to L&E and plug it in when needed.  No chance of resistors catching fire...

... but other things (including people) could be adversely affected if there is a poor earth connection in the supply! A resistor having a high value (relative to a cold incandescant filament) will reduce but not eliminate those problems.

[Jeroen3]

I found an schematic displaying the safest and cheapest method to test an RCD with outcome "yes/no".
https://www.tlc-direct.co.uk/Book/8.6.3.htm (schematics always say more than a thousand words, right?  )

No need for earth, or expensive additional devices. If you need a test result of "within spec" or "not within spec" you'll need a special tester.