IR Test

[imranayaz]

In the o.s.g page 96 - it states the minimum values accepted for insulation resistance, then it goes onto say in the paragraph below though they are the minimum accepted values if you get a reading of less than 2M ohms there is a defect in the circuit. I don't understand how? Can anyone explain?

 

[trev]

Anything over 2 meg is good to go. Under that you need to investigate.
It comes from EICRs really, an installation deteriorates over time. There's no way on earth I'd walk away from a brand spanking new install which gave me 2 meg on IR mate.

 

[imranayaz]

So what's with the minimum accepted values? I understand that you should get a higher reading on IR on a new install that's mentioned in guidance note. Should I ignore the minimum and make not of anyhing over 2MOhm is good anything below further investigation is required.

 

[trev]

Well there has to be a mimimum mate, you have to draw the line somewhere and say that one side of the line is OK and the other isn't. 2 million ohms is still a fair old whack of resistance but it is a sign of deterioration.
Personally if I was doing an EICR and got that reading I'd be recommending further investigation to the customer anyway

 

[imranayaz]

Question. Explain why it is necessary to determine the earth fault loop impedance value of a
circuit?


My answer. So that the breaking capacity of the protective device is greater than the prospective fault current.


Is is this correct?

 

[Simonslimline]

Question. Explain why it is necessary to determine the earth fault loop impedance value of a
circuit?


My answer. So that the breaking capacity of the protective device is greater than the prospective fault current.


Is is this correct?

To make sure disconnection times will be met.

 

[imranayaz]

When voltage sensitive equipment cannot be removed when performing an insulation resistance test , using the method of connecting the two line conductors together (line and neutral) and testing with cpc. Is the voltage sensitive equipment not getting damaged because it is not getting full 500v's?

Can anyone explain?

 

[Marvo]

Think about achieving disconnection.

*edit* Way too slow

 

[Marvo]

If voltage sensitive equipment is on a circuit you shouldn't be IR testing at 500v.

 

[Marvo]

Your insulation tester actually generates a high voltage during the test. You need to ensure that sensitive equipment never receives this test voltage across the L+N because damage could occur. The best way to ensure that a voltage can't develope across the L and the N is to connect them together and that way they will always assume the same voltage.

 

[trev]

When voltage sensitive equipment cannot be removed when performing an insulation resistance test , using the method of connecting the two line conductors together (line and neutral) and testing with cpc. Is the voltage sensitive equipment not getting damaged because it is not getting full 500v's?

Can anyone explain?

What you're doing is testing both live (sorry, cannot get used to lines. And they're still phases to me) and neutral against the cpc at the same time. As Marvo says though to be on the safe side you should test at 250v
Lots of things don't like 500v up em that's why on an EICR I always do a 250 test before shoving 500 up.

 

[Simonslimline]

What you're doing is testing both live (sorry, cannot get used to lines. And they're still phases to me) and neutral against the cpc at the same time. As Marvo says though to be on the safe side you should test at 250v
Lots of things don't like 500v up em that's why on an EICR I always do a 250 test before shoving 500 up.

Same here, although the last place i worked the guy laughed at me for doing it like this. More fool him i say. Better to be safe and not damage any sensitive equipment during the test.

 

[Marvo]

Another problem with higher test voltages is that sensitive equipment nearly always has surge arrestors built into it and these will give you low IR readings at higher test voltages and cause you to think there's a fault when actually there isn't.

 

[imranayaz]

Isit just for disconnections times
to be met?

 

[imranayaz]

Isit just, so that the disconnections times
can be achieved?