Zs

[optician]

Hi, hopefully a quick question for you guys.
Can you take a Zs reading with electrical equipment
still connected to the circuit?

 

[Jurassic Spark]

Loop tester springs to mind.

 

[optician]

Sorry, not making myself clear.
Loop tester plugged in, other electrical equipment e.g. computer, still connected to circuit. Can you still take zs reading? with out damage to computer.

 

[spamwize2]

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

 

[12345aob]

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

Are you saying that Zs testing is dangerous to computors but IR testing is not?

 

[uksel]

would the test current run through the equipment plugged in?

considering the path of least resistance i would assume that the equipment would not be affected

thoughts...?

 

[LukeScotty]

i think uksel is right, all a zs reading does is find out the potential difference

 

[uksel]

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

 

[optician]

Glad to see me question has got you thinking. Here are my thoughts, which are not necessarily correct.
Insulation resistance, which is not what my question was about, is 500v (dc I think) and will fry any semi conductors, computers and dimmers included when tested across phase and neutral.
Zs measured with a loop meter, measures impedance (not potential difference) of the phase and earth loop under fault conditions, a suitable low reading compared with the regs ensures quick opperation of the MCB.
A good earth path is required, so I think, the current generated by the loop tester follows the quickest route to earth, therefore, not affecting any computors or the like, still connected to the circuit?
I would be grateful if any one can confirm this.

 

[Spudnik]

Why would you want to test a circuit with items still connected?

Dont see it as a problem however, im not sure i would risk it myself, regardless.

 

[uksel]

perhaps if the equipment was essential and the client did not want it turned off under any circumstances

although by performing the test you risk tripping the RCD/RCBO's anyways which would mean a wasted effort and an unhappy client.

with split boards you could just test the side not containing the live equipment, Zs readings would suffice from a furthest point, light etc.

If it is essential that supply is not cut off from the equipment, why not try a few test runs at home, see how you get on?? perhaps you could consider it damage limitation

 

[WarrenG]

I would have thought that all the equipment would have already been unplugged for the previous tests?

Loop testers will place a Line/earth fault on the circuit under test - 23Amp Pulses. But only for a max of 40ms.

IT equipment can also have a leakage to earth, so I guess for a true reading it would be better to unplug this type of kit?

According to NICEIC for circuits upto 25mm / 50Hz it is perfectly acceptable to determine Zs by the formula Zs = Ze + R1 + R2

 

[rallyandy]

I agree with warren, better to disconnect equipment especially computers as warren says if the rcd trips you might harm the computers anyway, and also what if there is a fault on the circuit that then passes the test current through the equipment

 

[uksel]

loop testers pulsate an output of 23Amps?

are you sure that is right? it sounds a little excessive, would 23mA not be a more realistic figure?? I'm not saying you are wrong as i don't know, it just does not sound right to me

if it is then you learn something new every day eh

 

[jeremy]

Old Z testers used to use high currents 23a doesn't sound unreasonable but any decent modern tester 1653 eg uses a really low current under 15ma this wont trip your RCD unless it's faulty. Your Zs is testing the earth fault loop. Nothing goes up the neutral it's used to pwer the test. if there are alot of PCs on the circuit they rely on earth leakage current to work correctly so the accumulation of Zs and PCs could trip the RCD. Tell him to switch the things off. hope this helps or at least makes sense

 

[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'

 

[andyb]

A loop tester will test the earth loop impedance, it does this by inserting a resistor across line & earth, it cannot damage any other equipment connected to that circuit.