Surge protective devices (SPD) testing with 250 V dc?

[ericmark]

I can see that when testing insulation resistance you may want to use DC so it is not showing current through a capacitor to get the 1MΩ required under the old BS7671:2008 rules. However for practical purposes, what we want, is to be sure when a RCD is controlling the circuit it will not trip the RCD so in real terms it would be better to test with 250 V ac at 50 Hz so any surge protective devices (SPD) which could cause enough leakage to cause the RCD to trip are highlighted?

Odd at 500 and 1000 volt the old BS7671:2008 does not say DC or AC only at 250 V is it stipulated dc, my old meter does not have an option to test with AC but it is old, and at that time 1980 we did not have that many RCD's around, not seen a AC/DC switch on a meter, but maybe today that is a required option?

 

[JohnW2]

I can see that when testing insulation resistance you may want to use DC so it is not showing current through a capacitor to get the 1MΩ required under the old BS7671:2008 rules. However for practical purposes, what we want, is to be sure when a RCD is controlling the circuit it will not trip the RCD so in real terms it would be better to test with 250 V ac at 50 Hz so any surge protective devices (SPD) which could cause enough leakage to cause the RCD to trip are highlighted?

I'm a bit confused by what you're saying. The purpose of IR testing is surely to detect defects in cables etc., not to determine whether there things connected to the circuit which, during normal operation, might result in enough "leakage" to trip an RCD??

Kind Regards, John

 

[ericmark]

I'm a bit confused by what you're saying. The purpose of IR testing is surely to detect defects in cables etc., not to determine whether there things connected to the circuit which, during normal operation, might result in enough "leakage" to trip an RCD??

Kind Regards, John

This is the whole reason for the question, I like I am sure many others, have been called to RCD tripping problems, the insulation tester be it stand alone or built into a PAT tester is one of the main tools used to hunt for items which can cause the RCD to trip. The use of DC for most tests is great, as it will not leak due to inductive or capacitive links, and so in the main we want to use DC. But if you know you have a leak they also you want the tester to show where that leak is, some testers can actually measure leakage current, however the RCD needs to hold in while testing, you can't test without powering the item up, and where timers bring in pumps or heaters at some time through a cycle one would need to watch the meter for a long time before it completed a full cycle, so again you want to be able to test while dead.

There is it seems a cheap meter on the market which some one had bought and found they were getting different results to a Mega with a filtered socket extension but with a test resistor the same results, most likely reason is an ac component to the test voltage, first reaction was you don't want an ac component, however on further thought not so sure.

 

[JohnW2]

This is the whole reason for the question, I like I am sure many others, have been called to RCD tripping problems, the insulation tester be it stand alone or built into a PAT tester is one of the main tools used to hunt for items which can cause the RCD to trip. The use of DC for most tests is great, as it will not leak due to inductive or capacitive links, and so in the main we want to use DC.

Indeed.

But if you know you have a leak they also you want the tester to show where that leak is, some testers can actually measure leakage current, ....

Is it not a question of using the appropriate tool? IR testing is surely primarily intended for what it says on the tin - testing the resistance of insulation (nearly always of cables) - and, as you have said DC is desirable for that, since it avoids complications due to capacitive/inductive paths. Indeed, one usually disconnects as many 'loads' as possible when undertaking IR testing, doesn't one? However, if one wants to investigate loads for 'leakage' (possibly capacitive), one really has to take a different approach - often, in the first instance, the pragmatic approach of disconnecting the load and determining whether the RCD still trips.

.... however the RCD needs to hold in while testing, you can't test without powering the item up, and where timers bring in pumps or heaters at some time through a cycle one would need to watch the meter for a long time before it completed a full cycle, so again you want to be able to test while dead.

If potential causes of leakage (like pumps) are only connected by timers, then one could only test them 'while dead' if one dismantled the appliance and got directly at the connections to the pump (or whatever).

Kind Regards, John

 

[ericmark]

Not sure if the 750 V ac button is to test installation or to test voltage, however having the option to use AC would be an advantage for fault finding, I have in the past powered items of low current using the inverter in the car so I can test if it passes current to earth when powered with AC when I could not get a RCD to hold in. Most filters do not allow enough current to earth to trip a RCD, however when faulty either by design fault or damage we have nothing we can really use to test leakage without powering the item, not looking for an accurate reading just something to use for fault finding.

 

[AdrianUK]

Isn't the answer here to differentiate between the fixed wiring & an "appliance"?

Using your pump as an example, isn't the pump still an appliance, even though it may well be connected to the fixed wiring without the use of a plug & socket?

If we take that line, then the fixed wiring can be tested with the insulation tester & the appliance with a PAT tester. Most PAT testers (excepting the real, basic, entry level devices) will monitor the actual earth leakage current of a device, while operating normally, in real time. Surely this is the tool to decide if a device is leaking more than it should? I've also powered the PAT tester through an isolation transformer so that I can power up a suspect device & read its leakage without taking out an RCD.

 

[JohnW2]

... however having the option to use AC would be an advantage for fault finding, I have in the past powered items of low current using the inverter in the car so I can test if it passes current to earth when powered with AC when I could not get a RCD to hold in....

Fair enough. However, if you test, and confirm satisfactory operation (without the load concerned connected), of the RCD "which will not hold in", then you essentially get the same information by observing that the RCD trips when you connect the load/appliance as you would by actually measuring the leakage current of the device.

If, for some reason, you actually want to measure the leakage current, then, as I said before (and Adrian has now echoed) you should use the correct tool, and that's not really an"insulation resistance tester".

Kind Regards, John