Zs

[WarrenG]

I use a Megger MFT1552 which has both a no trip loop test at 15mA and a HI Loop test at 25A.

I believe that the IT equipment uses filters which are used to protect against interference and for power surges, depend on a connection with earth for the filters to operate?

 

[uksel]

Lo and behold


I have learnt something new

 

[coxy]

Hi All,
Connecting line & neutral together and tested to earth will not damage equipment during insulation resistance test.Never known damage to computers etc when left plugged in during loop test.

 

[uksel]

why would you connect line and neutral together when doing an IR test??

 

[jeremy]

If result is within limits then with one test you have tested live - earth and come away happy

 

[tigerpaul]

I've done hundreds and hundreds of loop tests on all kinds of circuits, and never had any problems of the kind your describing.

To be fair, i've IR'd Phase-Neutral on 500V by mistake more than a few times, and got away with it. Maybe I've been fortunate up until now!

 

[jeremy]

There is no problem IR testing P-n but by joining P-N together on a PIR and testing to earth you're cutting out a step. As the book says P-N, P-PE, N-PE. If you do P/N-PE you only do one test as long as it all measures out ok (it should on a pir as it has been working for the last 75 years since the last one!!)

 

[uksel]

really? i've always done them seperately, way i take it, if you test P/N - PE, there could still be a dead short between Phase and Neutral and without doing the seperate tests you'd not know.

is this wrong?

 

[jeremy]

If you are doing a pir then hopefully the breaker would have gone if there was a problem. On a new install check every circuit. but if the circuit has been in use since the dawn of time then a little discretion should be used. Testing is not just about taking a reading and writing it down but taking 5hitloads of readings writing them down, and then looking at them to see if they match up. Do everything by the book and you'll never be "wrong" but do it all with an understanding of WHY and it'll happen a lot quicker.

 

[uksel]

Fair comment. I suppose that's where competence plays it's part, there's a big difference in being able to think about things as compared to just doing things and going through the motions eh!

Hi All,
Connecting line & neutral together and tested to earth will not damage equipment during insulation resistance test.Never known damage to computers etc when left plugged in during loop test.

if you connect line and neutral, which jeremy has explained to me is perfectly reasonable, then it wouldn't harm any equipment plugged in because there would be no potential voltage to attract current from line to neutral. same theory as equipotential bonding

^^ does that sound about right?

 

[jeremy]

Spot on!! If P is 1GV and N is 1GV their potential difference is still a big ZERO so no current will flow ( between them, however at 1GV I wouldn't want to be in the same room!!!). I really hope this has helped with a bit of testing theory

 

[maddfridge]

hi all

then i have been lucky !!

tested zs at sockets with kit plugged in could not power down various reasons
obiviously no ir tests done

cheers

will think about this in future though

 

[simopesh]

if u talking bout loop test then no.if u talking bout insulation resistance yea

Set the isolation resistant tester to 1000 volts instead of 500.

 

[jeremy]

My 1653 doesn't have an isolation resistant tester. What should I do? And I don't understand the 1000V setting. Please help. yours expectantly, Jeremy.

 

[uksel]

My 1653 doesn't have an isolation resistant tester. What should I do? And I don't understand the 1000V setting. Please help. yours expectantly, Jeremy.

^^ Please take a note of 'the perfect response'