Figure of 8 tests

[Jupiter01]

As you imply, even if applied between L&N, 250V DC is very unlikely to damage anything used to experiencing 230V (RMS) AC. However, we're talking (well, I think we are!!) about the situation with L and N joined together (and testing between them and earth - in which case, as I said, I can't see any credible test voltage damaging anything other than possibly input filter (L-E and N-E) capacitors.

Kind Regards, John

Can I please check that an IR test can be performed by disconnecting the ring legs from a socket - whilst everything is connected in the CU?

This saves having to locate these in a busy CU.

I think the recommendation is to test at 250v across L&N to Earth when we are concerned about connected or vulnerable loads?

 

[Harry Bloomfield]

As you imply, even if applied between L&N, 250V DC is very unlikely to damage anything used to experiencing 230V (RMS) AC. However, we're talking (well, I think we are!!) about the situation with L and N joined together (and testing between them and earth - in which case, as I said, I can't see any credible test voltage damaging anything other than possibly input filter (L-E and N-E) capacitors.

Kind Regards, John

and the input filtering will usually be rated for 400v, with no electrical difference between the L and N input pole.

 

[JohnW2]

Can I please check that an IR test can be performed by disconnecting the ring legs from a socket - whilst everything is connected in the CU? This saves having to locate these in a busy CU. I think the recommendation is to test at 250v across L&N to Earth when we are concerned about connected or vulnerable loads?

As I've said, no type of IR testing (between anything and anything else) at 250V really ought to do any damage to anything. I suppose it is just about conceivable that something happy with 230V (RMS) AC (i.e. about 325V peak) might get upset with 250V DC, but I really can't think how/why that would be the case.

As I've also said, then if L and N are joined together, with IR tesfing undertaken between them and E, then there should not generally be any risk to loads at any credible test voltage (i.e. 500V or 1,000V) BUT, if the connected equipment had L-E and/or N-E input filter capacitors, those capacitors could be killed with 500V, and more probably with 1,000V.

So, the bottom line is that using 250V I don't personally think there is any significant risk of doing damage, particularly if L and N are joined together..

However, bear in mind that even if 250V doesn't do damage to loads, the presence of connected loads could affect the IR reading that you got.

I am no authority, so others may have different views.

Kind Regards, John

 

[RandomGrinch]

As far as I can see, the only possible exception (although not at 250V) would be when there are L-E and/or N-E 'filter capacitors' in the equipment, which could be damaged if they were subjected to excessive voltage (which, in practice, would mean 500V or above).

I can confirm, a 500V d.c. insulation test can damage items with mains input filters
PAT'ting of potentially sensitive equipment is now set to 250V d.c. to avoid damage to academics personal hifi equipment! - if it's there we PAT it!

 

[JohnW2]

and the input filtering will usually be rated for 400v, with no electrical difference between the L and N input pole.

Quite - which is why I've said that a test voltage of 500V, or higher, could damage such (L-E and/or N-E) filter components, even with L and N joined together. ... and, even if the test voltage did not damage the filter capacitors, leakage through them could result in artificially low IR readings.

Kind Regards, john

 

[RandomGrinch]

... and, even if the test voltage did not damage the filter capacitors, leakage through them could result in artificially low IR readings.

Kind Regards, john

Yup!
Extension leads normally get the extreme PAT treatment from us; 25A earth bond, 40V a.c. polarity check and a 500V d.c. insulation test.
Filtered extensions, will instantly fail the 500V insulation test, by doing what they are designed to do!

 

[flameport]

Can I please check that an IR test can be performed by disconnecting the ring legs from a socket - whilst everything is connected in the CU?

Only if the circuit is connected via a double pole RCBO or MCB.
Otherwise the neutral will still be connected to all of the other neutrals for the other circuits.

 

[Jupiter01]

Only if the circuit is connected via a double pole RCBO or MCB.
Otherwise the neutral will still be connected to all of the other neutrals for the other circuits.

Since most MCB’s are single pole and are using the shared neutral bar, tracking down the ring legs in a busy CU would be an inevitable part of the IR test.

 

[JohnW2]

Only if the circuit is connected via a double pole RCBO or MCB. ... Otherwise the neutral will still be connected to all of the other neutrals for the other circuits.

Since most MCB’s are single pole and are using the shared neutral bar, tracking down the ring legs in a busy CU would be an inevitable part of the IR test.

I suppose it depends to some extent how 'pragmatic' (per 'common sense') you are prepared to be, and how 'lucky' you are.

There is no way that having neutrals of other circuits connected whilst one is undertaking an IR test can increases the IR measurement that one gets. Hence, if one undertakes a test under those conditions and gets a result which is high enough to be 'satisfactory' for the circuit, then one can be confident that the IR of the 'circuit under test' is also satisfactory'.

If the test under those conditions produces a result which is 'too low', then one would have to repeat it with the circuit's neutral disconnected from those 'other neutrals' (whereupon the test result for the circuit may be 'high enough').

Kind Regards, John