In the 16th ed version of GN3 it says that for Pscfc you connect Green and black leads together to Neutral and for Pefc the 3 leads go in their respective place.
The new GN3 states that manufacturers instruction should be followed. I have been using a Megger 1553 and can't find anything in the instructions about this. The 3 leads are only needed on the no trip setting. Does anyone know if the connections are the same as stated in the old GN3?
Thanks in advance.

The third lead of a three-lead loop impedance tester is only to allow the instrument to be powered from the circuit under test.

Where a neutral is available, it should be connected to the neutral, where a neutral is not available it may be connected to earth.

The manufacturers instructions for your specific test instrument may refer.

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Ross Currie TMIET

Joining the N & E together can trip an RCD at half the rated I, when carrying out a Zs (just an additional note)

Regards
Carl.

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Regards
Carl

Thanks for the reply. I'm ok with what you've said about Z testing but wasn't sure about fault current. The fault current would be tested at the DB so there would be a neutral.

I've looked at the Megger instructions and can't see any guidance apart from you only need 2 leads for a high current test and all 3 for a low current test.

A prospective fault current test is just the same as a loop impedance test, except the instrument calculates internally the prospective fault current from the supply voltage and measured loop impedance.




The Megger 1553 seems to be an exception to the rule regarding two-lead and three-lead instruments.

It would appear that for a no-trip test (L-PE) you do need to use all three leads, connected as described on page 10 of the instrument manual. You should then be able to measure loop impedance and prospective fault current (L-PE).

To measure prospective fault current (L-N) you need to use only two leads and connect as per the instructions on page 11 of the manual. You should then be able to measure prospective fault current (P-N).

Hope that helps - its a little more complicated than most!

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Ross Currie TMIET

I don't own one of these testers(shame) but I have used one. Also I have seen the Megger Youtube videos. Worth a look. I believe you can do a 2 lead high current test with the instrument. If all else fails read the instructions.

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John Peckham

http://www.astutetechnicalservices.co.uk/

I can see why they may have changed the diagrams and text in GN3 with respect to loop impedance and prospective fault current testing.

The diagrams and text in the previous edition of GN3 were correct (as far as I can recall) for the Robin type three-lead instruments, but are not so for the more recent two-lead instruments such as the Megger LTW315 or as we have discovered the Megger 1553 which appears to use two or three leads depending on whether it is desired to carry out a no-trip or high current test.

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Ross Currie TMIET

Yes, thats correct but if you have an rcd upstream then you need to do a no trip test and all 3 leads need to be connected. I just wasn't sure how the 3 leads connect for Pefc and Pscfc.

For PEFC connect the 3 leads to Line, neutral and earth. For PSSC connect the Red to line and the black and green to neutral.


I have the Megger LTW which does a 2 lead no trip test as well as a high current 2 lead test. Best loop tester in my armoury and I have lots.

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John Peckham

http://www.astutetechnicalservices.co.uk/

My Megger CM500 does the same - both non-trip and high current tests using a two wire connection - in fact it doesn't know what a three wire loop test is.

John, how stable is your LTW on <1 ohm measurements in non-trip mode? My CM500 varies by about 0.2 ohms and seems to always give a lower reading than the high-current measurement (for what it's worth).

Thanks John, so it as shown in the old GN3 as in my original post. Why don't the manufacturers instructions show this?

That depends on the meter, could just be a case of connecting the 3 leads to L/N/E and selecting the required test on the meter.

Regards

Try and forget the various diagrams from GN3 and read pages 10 and 11 of the Megger 1553 manual - its all in there!

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Ross Currie TMIET

Thanks for the replies Ross. Its probably me being thick but where on p10 & 11 does it say how the leads are connected to do either PEFC of PSCFC when doing a no trip 3 lead test?
All I can see is it show where to connect the leads into the meter.

My apologies, it is actually necessary to consult pages 9-12 of the instruction manual for the full information.

For a no-trip prospective fault current (Phase-Earth) test you need to use all three leads as shown at 'Option 1' on page 9, connected to the circuit under test as follows:

GREEN - Earth
BLACK - Neutral
RED - Phase

This is the same connection as for a no-trip earth loop impedance test as described on page 10. Connected as above, you can then carry out a no-trip earth loop impedance test or a no-trip prospective fault current (Phase-Earth) test.

To carry out a prospective fault current test (Phase-Neutral) you only need to use two leads, the (RED and GREEN) connected to the instrument as per Option 1 on page 10. Note that this is different from the Option 1 shown on 9! Connect to the circuit under test as described on page 11:

GREEN - Neutral
RED - Phase

Note that you need to use the 'PFC Hi' high current setting to measure prospective fault current (Phase-Neutral) - you don't need to worry about no-trip for tests (Phase-Neutral).

Clear as mud?

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Ross Currie TMIET