Zs and IR pitfalls

[tomlonsborough]

Hi folks,

I'm still fairly new to testing having just passed my 2391 and just looking for a bit of advice if poss..

Quick bit of background - I'm carrying out EICRs on a wide range of single and 3 phase boards, inc. motors, lighting, power, micc, singles, swa etc. Any general pointers/tips would be greatly appreciated!

Regarding IR testing, other than delicate electronics, fluorescent ballasts etc. is there anything else you would be testing at 250V or LN-E? Or any pieces of kit you might ordinarily get a strange reading for? For example I came across a strange 30M Ohm reading for a mechanical thermostat the other day - is this normal?

Also regarding Zs measurements, I'll be testing lighting circuits where it's not practicable to be removing all lamps - this hasn't been an issue for fluorescent lighting circuits, but testing circuits with LED lamps in situ and energised has caused the Megger to trip, even on the no trip setting. Can anyone shed any light (no pun intended) on this?

Many thanks in advance!

Tom

 

[telectrix]

with the LED luminaires, maybe wander lead and get a R2. then in conjunction with a calc. value for R1 (same as R2 if same size conductors), use Zs = Ze + ( R1+R2).

 

[JAWS]

Theres a good piece here Tom regarding Megger and LED Zs testing which say what telectrix suggests. Loop testing and LED lamps: what's going on? - https://uk.megger.com/electrical-tester-contractor-edition/april-may-2017/loop-testingand-led-lamps-what-s-going-on

 

[tomlonsborough]

Thanks for the input chaps, much appreciated.

 

[Bobster]

Some thermostats will use a neutral for hysteresis. Your 30M ohm result could be that.

 

[tomlonsborough]

Some thermostats will use a neutral for hysteresis. Your 30M ohm result could be that.

Thanks Rob, that might explain it.

 

[Simon47]

Just a thought ...
With traditional pendant lampholders etc, it's easy to remove (or not fit initially) the lamps and be able to do L-N, L-E, and N-E IR tests. Now there's so many lamp types where this isn't the case, I wonder how many people either IR test the reels of cable beforehand, or IR test each individual cable before connecting the luminaires ?

 

[Risteard]

Regarding IR testing, other than delicate electronics, fluorescent ballasts etc. is there anything else you would be testing at 250V or LN-E?

For periodic inspection and testing I would only ever suggest testing between live conductors connected together to Earth. I would never test between live conductors unless specifically agreed to do so. I certainly wouldn't be recommending it.

 

[happysteve]

Steady and consistent IR readings of around 0.03MΩ or 0.04MΩ (testing between L+N tied together, and earth) might well be an RCD spur or RCD socket.

 

[arashabb]

Hi Guys,we use RCD+MCB in every circuit over here( TN-S system). what would be the max Zs? Do you refer to MCB table?

 

[Wilko]

Hi - yes I design based on MCB rating and it’s max earth loop impedance. The RCD is then for additional protection.

 

[arashabb]

Hi - yes I design based on MCB rating and it’s max earth loop impedance. The RCD is then for additional protection.

Thanks alot. It makes it more confusing as we have a TN-S system with additional local earthpit(s) in the property

 

[Marti]

A really useful thread. Gotta be more mis-conceptions about IR's than RCD and continuity combined. USB sockets and dimmers just adding to the confusion.
My query is:
Before a board change and with all RCD's & MCB's "off" I'll I.R. test the Live from the dead side of the breaker to Earth and Neutral having removed both from their respective bars.

Something I've always done but was suggested I'm wasting time removing both N and E. Personally speaking, I want a fault-free system before I remove the old board and inherit all the hidden issues.

What's yer thoughts guys?

 

[Risteard]

A really useful thread. Gotta be more mis-conceptions about IR's than RCD and continuity combined. USB sockets and dimmers just adding to the confusion.
My query is:
Before a board change and with all RCD's & MCB's "off" I'll I.R. test the Live from the dead side of the breaker to Earth and Neutral having removed both from their respective bars.

Something I've always done but was suggested I'm wasting time removing both N and E. Personally speaking, I want a fault-free system before I remove the old board and inherit all the hidden issues.

What's yer thoughts guys?

Disconnecting the cpcs from the Earthing arrangements for the installation is a non-compliance with BS7671.

 

[Pretty Mouth]

I remove N but leave E in the E bar - a circuit can have a fault to earth that isn't to it's own CPC. I find a L-N test of limited use for an installation that's already in use and doesn't have a known fault. More useful is linking L+N of a circuit, and testing between that and all other circuits in turn, looking for interconnections. Testing at 250v, with equipment in place and switched to on position, can help find borrowed neutrals etc.