Very Low Ze & Very High PFC

[gareth020]

Hi
So today i tested at the orgin of the installation and on my MFT 1552 Megger, i got a reading of <0.01 for Ze and >19.9 KA for PFC, it's a medium sized industrial unit and is fed from a substation about 30 meters away, so i expected it to be low. Obviously the client wouldn't let me switch off to disconnect the main earth, so i understand i would be getting parallel paths. But still does this not seem just a bit too low!?
As for the PFC, the main switch is 33KA rated and the mccb's inside the section board are 25KA so i guess they are all ok, but there are two Mccb's that are rated at 15KA, do i have to change these, or is it just the main switch KA rating i need to be concerned with!?
Finally My meter says greater than 19.9KA ...how do i find the actual value!? Or shall i just record this reading!?

Thank You

 

[AP Electrical]

ohms law v/R

 

[malcolmsanford]

There is an excellent man on here called IQ, and a thread about just this. So instead of hunting through the forum i'm going to do it the lazy way.

If you PM IQ I'm sure he will have a link to the thread.

Just though to put your mind to rest, yes the parallel paths will give you a false lower reading. Remember our MFT are not as accurate as to show a realistic reading at such low levels.

IQ concocted a very good way to do this using a wander lead as a resistance in series with you meter and conducting the test using the cable resistance of the wander lead, this will bring your meter into a much better parameter. You then of course need to subtract the resistance of the waander lead and that will give you your Ze.

Of course you can't get a true Ze with the parallel paths, was this a PIR or an EIC

 

[Chr!s]

Hi
So today i tested at the orgin of the installation and on my MFT 1552 Megger, i got a reading of <0.01 for Ze and >19.9 KA for PFC, it's a medium sized industrial unit and is fed from a substation about 30 meters away, so i expected it to be low. Obviously the client wouldn't let me switch off to disconnect the main earth, so i understand i would be getting parallel paths. But still does this not seem just a bit too low!?
As for the PFC, the main switch is 33KA rated and the mccb's inside the section board are 25KA so i guess they are all ok, but there are two Mccb's that are rated at 15KA, do i have to change these, or is it just the main switch KA rating i need to be concerned with!?
Finally My meter says greater than 19.9KA ...how do i find the actual value!? Or shall i just record this reading!?

Thank You

Firstly, i agree with Malcolm, check accuracy of your meter at that level, IQ has a thread where he explains a method for greater accuracy.

Secondly, you need to see what the protective devices lets through at that level of fault current, i would expect a good degree of current limiting so even the 15ka rated devices may be ok.

I would need more info, the type of protective device, an accurate PFC reading and the breaking capacities of the equipment.

 

[Electricalserv]

Firstly, i agree with Malcolm, check accuracy of your meter at that level, IQ has a thread where he explains a method for greater accuracy.

Secondly, you need to see what the protective devices lets through at that level of fault current, i would expect a good degree of current limiting so even the 15ka rated devices may be ok.

I would need more info, the type of protective device, an accurate PFC reading and the breaking capacities of the equipment.

I had thios on a domestic last week, posted a thread on it. IQ came up with some method of getting a better reading... Sub station is to blame..

 

[gareth020]

Thank for the advice guys.
Much appreciated.
I'll check out IQ.

 

[RogertheBodger]

Sticky: Higher-accuracy low Ze test procedure
Started by IQ Electrical, 1 Week Ago 10:40 AM

Hi Mate

This is the thread that will help you, it's in the 'Stickys', Inspection and testing forum.