Help with the meaning of tests.

[Luminous]

Hi, I'm a relatively inexperienced trainee electrician, especially when it comes to testing and I wonder if you could clear up a couple of things?

I'm currently on my level 3 2330, have my 17th ed and a couple of other bits and bobs.
I've only ever done any testing in controlled environments, but I've purchased a 2nd hand multi-tester which is calibrated and have been messing around with it (so to speak).
I understand the procedure and theory behind most of the tests but am at a bit of a loss as to the PSC test.
I've tested my Mum's house and don't understand a couple of the results. She has a BS3036 board and it's on a TT system. When I do the PSC test from a socket it (on 200A) gives a reading of OL (Out of Range/>50ohm). Is that right? I didn't turn it up to 2000A in case I blew something up!

What kind of readings should I be looking for on the various systems for PSC, and where would I find them in BS7671? and could someone explain in laymans terms what is PSC. or more rather why we would want either a low or high reading?


Secondly, when I did a Loop Test, most of the circuits were fine, except for 2 sockets in the kitchen which read about 130v when first plugged in and the 2 green lights for P-E and P-N were lit to show wiring is connected correctly, but a 3rd light which is red and has P-N in a box was also lit which is a warning showing that I should not proceed with the test. Fortunately I'd read the manual beforehand so didn't proceed.

Why would the volt reading be so low and what could be the fault.


Thanks in advance. I'm curious and would like to be able to inform my parents.

 

[ringer]

For starters, please re-read your post and double-check the current values you stated (200A and 2000A). You may wish to edit those values! Also, please state what brand and model of tester you are using, because different testers have different interfaces and test available. You state the voltage indicated 'when first plugged in' for the socket in question. Did the voltage indicated subsequently rise to full potential, or did it stay steady?

 

[Luminous]

It's a Robin KTS1620. The settings on the PSC settings are 200A, 2000A and 20kA.
It seemed to stay steady at 130v or there abouts. If I'm honest though, I only let it read for up to 10secs but it seemed to stay consistent.

 

[pushrod]

Not familiar with that tester but what i assume you are interested in is PFC (prospective fault current) of which there are 2 types PEFC (prospective earth fault current) and PSCC (prospective short circuit current) basically these are the max currents that can flow in either a short circuit situation or an earth fault situation. Basically you want these to be large enough values to ensure that the protective device will operate in the correct time. If these values were too big though that could also cause problems as they might exceed the rating of the protective device (usually an mcb and rated at 6kA). Hope that helps as a starting point

 

[pevvers]

As it's a TT system, the PEFC will be very low. Take your Ze and do a bit of Ohms law magic with it to find the value you should be looking for in the instrument, i.e. V=230, R=Ze, so V/R will be in the region of what your PEFC should be. Your PSC would be the line-neutral impedance in place of the Ze in the same formula....

The 130V at your sockets sounds like a loss of earth continuity at the outlets, and should be investigated ASAP...

 

[ringer]

....
The 130V at your sockets sounds like a loss of earth continuity at the outlets...

I would have thought it is a problem with the neutral giving a reading like that. With a loss of earth, you would still get 230V L-N.

 

[pevvers]

I would have thought it is a problem with the neutral giving a reading like that. With a loss of earth, you would still get 230V L-N.

Not if it's measuring for earth fault loop impedance...

 

[ringer]

Not if it's measuring for earth fault loop impedance...

I have a Megger 1552. When doing a Phase-Earth loop impedance test it first detects and displays the line voltage. Then it beeps. Then it starts the test. If his tester works the same way, then it is detecting only 130V before the test is started.

Though saying that, I always use the plug lead. Otherwise I have to connect green to earth and red to phase, so perhaps it is measuring phase relative to earth. But I still say that if there is loss of earth continuity then there will be no potential.

 

[pevvers]

I have a Megger 1552. When doing a Phase-Earth loop impedance test it first detects and displays the line voltage. Then it beeps. Then it starts the test. If his tester works the same way, then it is detecting only 130V before the test is started.

Though saying that, I always use the plug lead. Otherwise I have to connect green to earth and red to phase, so perhaps it is measuring phase relative to earth. But I still say that if there is loss of earth continuity then there will be no potential.

I use a Megger 1552 (actually it's an old school 1502 that has served me well!) as well. Unfortunately it's not as simple as "no earth, no voltage" as current always seems to find some where to flow on normal circuits, whether this be induced voltages, or voltage transferring from neutral to earth on other parts of the circuit I can only guess.

Every time I've found anywhere between 70-130V on an installation my first guess has always been no earth, and 100% of the time this has been correct.

 

[pevvers]

Is that right? I didn't turn it up to 2000A in case I blew something up!

Changing the range will not affect anything, as long as you stick to live test settings when working on live circuits!!

To get overange on a TT system PEFC on 200 ohms would mean an an external earth fault loop impedance of just over an ohm, which is remarkably good for a TT. Just switch it to the 2000A range and come back to us with what reading you get...

 

[IQ Electrical]

This reading would include the protective bonding conductors parallel paths from gas/water which is fine for a PFC measurement but can't be transposed to calculate Ze/Ra which should be measured without any parallel paths.

 

[pevvers]

Yes, but this fella doesn't really need to know the exact science between true Ze and PFC readings at the moment, as he's just playing with the instrument at the moment. Once he's got the measurement in range, then maybe it would be time to go a little bit further in depth with measuring Ze with just the earthing conductor, and PFC with all parallel paths in place, otherwise it's only going to confuse the guy further.

As I said, if he's overange on the 200 ohm range, he'll be looking at a maximum 1.15 Zs from wherever he's testing. If Zs is lower, then he needs to up the range until he can get a measurement.