Insulation Resistance

[thomo27]

Hi

I have been practicing some dead testing on my home CU prior to my level 3 C&G in Electrical Ins begins.

I carried out an Insulation Resistance test on the downstairs ring and was concerned to see a reading off 0.00 M ohms between Phase & Neutral , N & CPC was 4.96Mohm & Phase&CPC was 4.96Mohm.

0.00 would indicate to me I have made a mistake with the connections or the test but I had the 6 cables disconnected from the CU and my connection was via the leads with the crocodile clips on the end i.e. 2 Phases and 2 CPC'S , 2 N's and 2 CPC's and 2 phases & 2 N's.

Could the insulation resistance really be that low ?

Any ideas most welcome

Regards

Ste

 

[Spark123]

Make sure everything is unplugged, all FCUs (boiler etc) switched off - it is more likely to be a load across L-N which is causing the low reading. If you do a continuity reading what does it read?

 

[Qedelec]

Have you remembered to unplug ALL appliances?

 

[thomo27]

Hi

I unplugged everything that I could see although come to think of it I believe I missed the the microwave and the kettle


continuity for N = 0.43
P = 0.45
CPC = 0.96

 

[ricicle]

Uh-oh, I hope you have not destroyed the electronics in any of your appliances as some tend to not like 500V up them!!

 

[Spark123]

I meant continuity from L to N as this will show any resistive loads connected across.

 

[thomo27]

Continuity between L-N = 0.92

 

[Spark123]

Now try unplugging everything else you can think of remembering any hidden sockets for the likes of the cooker ign etc and try to get the figure to go up!!

 

[ebee]

First make absolutely sure you have actually unplugged everything and and switched off all isolators that might be attached to this circuit.
If you have installed the whole of the wiring yourself then this should be easy, if not then you can`t be certain what someone else might have done.

Next It`s always better to check first with less than 500 V , if your tester has a 250V test then do this first, not as a test in itself but just to reveal if any items are still connected, if no 250V tester available then you might use a multimeter instead.
Remember a 500V DC test could damage something designed for 230/240V AC

If you`re happy nowt is connected then do the proper (500V ) test.

My suspicion is you`ve forgoten to disconnect something or there is something connected permanently direct to the ring and there`s no isolator for it.

 

[dingbat]

Continuity between L-N = 0.92

So, think about this:

What appliance could have a resistance of less than 0.92 Ohms? If you use Ohms law you'll see that through a resistance of this value you'd draw some 250A... I doubt very much that your circuit protective device would allow this current to pass! You are measuring a short-circuit, possibly because of how you have made connections.

continuity for N = 0.43
P = 0.45
CPC = 0.96

Look at p.158 of your On-Site Guide and see how your resistance measurements relate to conductor lengths. Have you left P-N connected at one end?

Then have a good read of pages 62-82, with particular attention to pages 69 - 73.

Have fun!

 

[Spark123]

Continuity between L-N = 0.92

So, think about this:

What appliance could have a resistance of less than 0.92 Ohms? If you use Ohms law you'll see that through a resistance of this value you'd draw some 250A... I doubt very much that your circuit protective device would allow this current to pass! You are measuring a short-circuit, possibly because of how you have made connections.

Something inductive like a transformer

 

[plugwash]

If you use Ohms law you'll see that through a resistance of this value you'd draw some 250A.

That is true if the load is purely a resistive load. Many loads are not for two reasons.

Firstly many loads have inductive elements that make thier effective impedance frequency dependent. The primary of a large transformer will be a near short circuit at DC even if there is no load on the secondry.

Secondly many loads are temperature dependent, stick a multimeter accross a halogen bulb and the resistance you measure will be far far lower than the resistance at operating temperature.

 

[dingbat]

Something inductive like a transformer

That is true if the load is purely a resistive load. Many loads are not...

Of course, but given that Thomo is a beginner and looking at his r1 and rn values it looks very much like he's tried to measure IR between line and neutral with the other ends connected.

Also, it's a socket circuit that he's testing. Not that that precludes the existence of inductive loads, but it would be unusual for such loads to be connected via invisible, unswitch-off-able means.

But one of the most useful ways of tracking down faults that appear as a short, is to use the resistance value measured, convert it into metres of the cable concerned and then try and narrow down the location by measuring at different points in the circuit. My suggestions were to get him thinking about what his readings actually meant.

 

[RF Lighting]

Also, it's a socket circuit that he's testing. Not that that precludes the existence of inductive loads, but it would be unusual for such loads to be connected via invisible, unswitch-off-able means.

Burglar alarm

The socket which FingRinal installed behind the fridge without an accesible isolator?

 

[BS3036]

Doorbell transformer, shaver socket...?