Confused by this question - help?

[MK1]

I’m revising for an exam and they’ve given me a multiple choice question paper but no answers (helpful!). Can anyone help me out with this one?

An insulation resistance test between live conductors on a single-phase circuit has been carried out and gives you a reading of 0.00M ohms. State one likely condition that could cause this value?

a) A resistive fault has occurred between conductors and water is present
b) A short circuit of negligible impedence between L and N
c) A short circuit of negligible impedance between L and cpc
d) A healthy circuit

 

[Silly Sausage]

Well, it isn't d!

 

[telectrix]

e) there is a load connected to the circuit?????

 

[MK1]

I figured it wasn’t d!

But other than that, I’m baffled! And I’ve written the question exactly as it’s been asked...

 

[ruston]

State one likely condition that could cause this value?

 

[Andy5678]

It’s not A as you don’t no if there is water there by testing.
And it’s not C because you are testing between live conductors.
It’s not D as a good reading should be over 1 Mohm.

So must be B

 

[MK1]

Thank you! I thought it was the most likely answer but I wasn’t sure. Really appreciate that!

 

[telectrix]

but it's a stupid question 0.00Megohms could be a residstance of anything uo to 999k ohms.

 

[johnduffell]

but it's a stupid question 0.00Megohms could be a residstance of anything uo to 999k ohms.

999k would be 0.99M even if rounded down assuiming the meter really does have that resolution and accuracy.
however you have a good point, could would be anything up to 9k so less than 30mA so as suggested above could be anything from a short down to a relatively small load e.g. <10W light bulb
Having said that we're only after one likely condition but doesn't give all the info: if it's testing after initial install there are less likely to be loads on the circuit and more likely to be faults.
If it's testing on an EICR that was recently energised it is mre likely to be the load.

 

[telectrix]

i stand corrected. never could sort that decimal point and 10 -6 crap ever since we stopped using feet and inches..

 

[Deleted member 26818]

This question is asking whether you know that neutral is a live conductor.

 

[MDJ]

Tel still uses the Abacus to work things out, please go gentle on him

 

[littlespark]

The only thing that can give a 0.00 reading is your test leads.... after you've nulled them

 

[johnduffell]

The only thing that can give a 0.00 reading is your test leads.... after you've nulled them

0.00 what? megohms or ohms?

 

[littlespark]

Mohms, as in the OP... but I see now my mistake.

 

[Wilko]

Yes Spin and Andy are correct - recall (look up) the definition of live conductor and go from there.

 

[static zap]

but it's a stupid question 0.00Megohms could be a residstance of anything uo to 999k ohms.

..maybe just under 9k but then who knows what will round to 0.01...
then there are the 1-or-2 lowest counts of some meters
Has anyone got a 4k7 / 8k2 resistor rated 250V to hand ?
(any nit pickers -who can be -arsked- )