Insulation resistance query

[tester11]

Hi, quick question for you all, do u think it is posssible to have an ins res reading of 0.02 on live earth of a lighting circuit which includes em. lighting but neutral earth is above 10 megohms ? There is low voltage track lighting also installed on the circuit.

Also can anyone explain the differnce between connecting live and neutral together and testing to earth or testing live earth then neutral earth seperately ?

Cheers

 

[davesparks]

0.02 what?
Yes of course it is possible, but the loads are irrelevant as you will have disconnected them to carry out these tests. The only test you would carry out with loads connected is with L and N shorted and tested to earth.

 

[tester11]

The loads are still connected as its a eicr in commercial property so not feasible to remove all lamps etc. That's why I find it strange that neutral earth isn't 0.02 MEGOHMS aswell ?? And also why I ask is L+N tested to earth different from testing L+E and then N+E seperatley ?

 

[davesparks]

The loads are still connected as its a eicr in commercial property so not feasible to remove all lamps etc. That's why I find it strange that neutral earth isn't 0.02 MEGOHMS aswell ?? And also why I ask is L+N tested to earth different from testing L+E and then N+E seperatley ?

Being a commercial eicr doesn't automatically mean loads cannot be disconnected, in fact it is sometimes very easy as most lighting tends to be on plug in connections. You shouldn't be testing with L and N separated with the loads connected as you could damage them, this is basic test procedure you should know if carrying out periodic testing

Why do you find it strange? A fault between L and E won't necessarily show up on a N to E test.

 

[Marvo]

0.02 what?
Yes of course it is possible, but the loads are irrelevant as you will have disconnected them to carry out these tests. The only test you would carry out with loads connected is with L and N shorted and tested to earth.

Even with L+N shorted there are certain anti-surge arrangements in things like electronic appliances and LED drivers etc that will pervert your IR readings unless you stay below 250v with your test voltage.

 

[tester11]

The lights are all high level it's not practical to remove the lamps. If u r getting 0.02mo live to earth then with the lamps in would you not get 0.02mo on neutral earth ??

 

[Marvo]

In simple terms most surge circuits would normally have an MOV between the L and E and another between the N and E so you see the same effect on both l-e and n-e IR tests but as MOV's get older or after they've been overloaded they start to break down and their leakage increases. This can usually be detected by the temperature they operate at during normal conditions and when they start getting hot it's time to replace or they fail completely. This breakdown would invariably be more prevalent in one of the MOV's so the leakage you see during IR tests would no longer be identical. To get a true IR reading you need to disconnect everything from the circuit. If the circuit has inaccessible items connected and is in use and hasn't been faulty (and you've performed a risk assessment) I guess you could calculate the IR at 240v by using a leakage clamp meter around the L+N as a live test then you can calc the IR from the actual leakage current.

 

[davesparks]

The lights are all high level it's not practical to remove the lamps. If u r getting 0.02mo live to earth then with the lamps in would you not get 0.02mo on neutral earth ??

Not necessarily, it depends what type of lamps they are, if incandescent then you'd expect it to have that effect but discharge lamps won't necessarily have that effect.

 

[davesparks]

The lights are all high level it's not practical to remove the lamps. If u r getting 0.02mo live to earth then with the lamps in would you not get 0.02mo on neutral earth ??

If you cannot disconnect the loads then you do not test without L&N shorted. This is a pretty basic part of the standard testing procedure