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