Insulation resistance

[Regs]

Getting really low insulation res. readings from my D.B.
Neutral bar to Earth bar 0.35 M.O
Line & Neutral to earth 0.35 M.O

So strange, R.C.D. DOES NOT TRIP!

Have tried removing main earth and this makes no difference

If any 1 could shed any light be great

Cheers.

 

[Marvo]

Firstly reconnect the earth immediately.

Can you give more detail on where and how you were doing the test and what tester and settings your using.

Does the RCD trip if you press the test button?

 

[jamesBoater]

Please correct me if im wrong but isnt the resistance still enough for the RCD to hold!

 

[Risteard]

So strange, R.C.D. DOES NOT TRIP!

Hardly strange, given that you have a resistance of 350,000 Ohms.

Therefore less than 0.0007A will flow. The RCD needs nominally 0.03A

 

[Regs]

Using a megger on 250v & 500v. Found one fault, when i disconnected the pond pump the reading has now gone upto 1.2 M.O. and have narrowed this second low reading down to the ring main between neutral and earth.

 

[Lucien Nunes]

Edit: ^^got there first

 

[Regs]

Hardly strange, given that you have a resistance of 350,000 Ohms.

Therefore less than 0.0007A will flow. The RCD needs nominally 0.03A

Ah O.K. cheers whats the formula you used here?

 

[jamesBoater]

simple ohms law

Ah O.K. cheers whats the formula you used here?

 

[44ALLAN]

Ah O.K. cheers whats the formula you used here?[/QUOTE]

No idea divided by several volts and adding 3 ohms over 2 amps............that should at least maim someone eh !! fgs?

 

[Des 56]

Hardly strange, given that you have a resistance of 350,000 Ohms.

Therefore less than 0.0007A will flow. The RCD needs nominally 0.03A

Slow down Risteard your getting to far ahead of the class

 

[Marvo]

Yep, the fault is too small to trip the RCD but I'd repair any fault under 1Mohm as a matter of course. If the fault has bee localised to a cble between 2 sockets then the cable may have been damaged or possibly there's damp in the sockets.

 

[Regs]

So when you do an insulation res. test before fitting an R.C.D. what minimum value would you be looking for?

Also would this now suggest my pond pump is on its way out or letting water in?

 

[Paultrim123321]

Mmmm Very Important Redhead?? Well that's how I remember it

 

[jamesBoater]

Surely with the 2391 under your belt this is stuff you should know !

So when you do an insulation res. test before fitting an R.C.D. what minimum value would you be looking for?

Also would this now suggest my pond pump is on its way out or letting water in?

 

[telectrix]

So when you do an insulation res. test before fitting an R.C.D. what minimum value would you be looking for?

Also would this now suggest my pond pump is on its way out or letting water in?

a reading over 1Meg. is acceptable, but it's good practice to investigate anything under 2 Meg. the pond pump could just be moisture in an adaptable box or outdoor socket.

 

[Regs]

Surely with the 2391 under your belt this is stuff you should know !

Yeah i know that of course. But if you add an RCD you are making the safety alot better you can't be held liable for a fault already in place. You could advise the customer and charge to correct it, surely?

 

[Regs]

a reading over 1Meg. is acceptable, but it's good practice to investigate anything under 2 Meg. the pond pump could just be moisture in an adaptable box or outdoor socket.

Yeah cheers, there is no adaptable box, i checked the socket. Insulation res. went up when i pulled the plug to the pump.

 

[telectrix]

Yeah i know that of course. But if you add an RCD you are making the safety alot better you can't be held liable for a fault already in place. You could advise the customer and charge to correct it, surely?

before adding an RCD you should test to see if there are any faults that will cause problems, along with earthing and bonding. you then factor the cost of fixing these faults as part of your quote.

 

[Marvo]

Any pumps but especially submersible types are prone to low insulation. Even slight wear on the shaft or seal coupled with the heating and cooling during normal operation and it's all over for the IR. If it's a small domestic pond pump then it's probably better to replace with new rather than attempt repairs.

 

[Octopus]

Doing an IR test with loads still connected is always going to end in tears.

 

[44ALLAN]

Hot motor /cold motor............summer /winter ambient temp lol.
Appliances just switched off or been standing for several hours.

The inexperienced will see faults that aren't there if their comparing results from previous tests

 

[Regs]

Doing an IR test with loads still connected is always going to end in tears.

Yeah i thought the load was removed so you don't fire 500v down your line and neutral and destroy items. Didn't realise, to be honest that items not in use but still plugged in would also still be a problem.

 

[LittleHelper]

Test between all conductors seperatley!! Probably a load!

 

[Brightspark2]

Check for more loads that are connected...or neons...FCU's etc etc

 

[Guest111]

Instead of disconnecting the main earth to the board,try disconnecting the L N E of the circuit you are working on and test them,it's just possible that even though you have disconnected the main earth that the bonds to gas and water may be affecting your readings.

 

[Regs]

O.k. forgot in 1 bed room there was surge protector under the bed, removed this and instantly gained from 0.36 to 1.6 M.O. Also found that unplugging my monitor for my mac gained another 1 meg to total 2.6 M.O. I'm happy for now with that, guess the rest could be the dishwasher or washing machine.
Again would this imply monitor or lead on way out?

 

[telectrix]

a lot of electronic gear has earth leakage. always disconnect before testing. as cpl. jones famously said " they don't like it up'em".

 

[Regs]

a lot of electronic gear has earth leakage. always disconnect before testing. as cpl. jones famously said " they don't like it up'em".

So as long as the cause of low insulation r. is not the cable insulation, then it doesn't matter if the insulation reading drops right down to 0.1M.O. The reason has been investigated and would still be acceptable of course then?

I take it some items of equipment are more prone to leakage than others then

 

[Marvo]

What you've discovered is that the earth is functional as well as protective. Depending what test voltage you use to mega test all surge arrestors will show as low insulation.

 

[Regs]

What you've discovered is that the earth is functional as well as protective. Depending what test voltage you use to mega test all surge arrestors will show as low insulation.

Now you mention it at 250v reading was 0.9 Mohm and 500v it read 0.36 if i remember correctly.

 

[Marvo]

A surge arrestor would see 500volts as a surge which it would allow at least a portion of the waveform to pass safely to earth. It's possibly a good job there were surge arrestors left plugged in because you're likely to damage electronics if you run around IR testing at 500 volts on circuits with appliances still connected. I'd count myself lucky and do some more reading on correct testing procedures if I were you.

 

[Regs]

A surge arrestor would see 500volts as a surge which it would allow at least a portion of the waveform to pass safely to earth. It's possibly a good job there were surge arrestors left plugged in because you're likely to damage electronics if you run around IR testing at 500 volts on circuits with appliances still connected. I'd count myself lucky and do some more reading on correct testing procedures if I were you.

Ive been taught and told as long as you don't fire down line to neutral at 500v it would be fine.

 

[Octopus]

This is basic stuff really.

Test IR at 250v first, 90%+ of low readings are due to appliances plugged in and always go round and unplug EVERYTHING, some sockets are SP isolators, as are some FCU's.

Never fire 500V at a circuit with really low IR unless you are 100% certain everything is unplugged.

And yes I mean test the circuit in question only, all 3 to each other.

 

[Regs]

This is basic stuff really.

Test IR at 250v first, 90%+ of low readings are due to appliances plugged in and always go round and unplug EVERYTHING, some sockets are SP isolators, as are some FCU's.

Never fire 500V at a circuit with really low IR unless you are 100% certain everything is unplugged.

And yes I mean test the circuit in question only, all 3 to each other.

If you still had a low reading after unplugging everything then last resort you would disconnect FCU's and once all avenues had been exhausted then you would have to put it down as cable insulation then?

 

[Marvo]

You assume that once you've unplugged everything and switched off any isolators you're seeing the IR reading for the circuit cabling and the actual sockets only but the problem you have is the hidden appliances you don't find such as TV antenna amplifiers in lofts etc will give false readings.

 

[Regs]

Going back to what i asked originally, i thought the insulation resistance test before fitting an rcd was for nuisance tripping not to see if you had over 1 M.O.