IR Testing Clarification Request

[markrutter]

Gents,
I’ve scanned back a couple of pages reading various entries on IR Testing but haven’t found the simplistic answer that I’m looking for. Apologies in advance if this question is posed on a regular basis by one of us rookies but my usual mentors are currently unavailable and this is really troubling me.

Before posing my query, I would add in my defence, I did raise this at the training centre that I recently left (with full scope Part P, Level 3 C&G 2382 17th Edition and 2393 Building Regs) and never received a definitive answer from the tutor but I still feel desperately apologetic for having to ask for clarity on this.

I’m about to add a socket to a ring main at home and thought, before I did it, I’d complete a Minor Works Cert to give me the before and after test results. All equipment was removed from all sockets (upstairs sockets so this was no trouble) but when I attempted to IR Test the circuit in question, obviously with main switch off at the CU, I got very low resistance readings, regardless of whether I tested line to neutral, line to earth or neutral to earth.

In the past I’ve seen an IR test affect other final circuits on the same board (eg. LED on shower pull switch light up when not testing this circuit) so am unclear on how to limit the 500V to the circuit in question. Should you just open all the MCB’s other than on the circuit being tested? There’s a 100mA RCD upstream (covering the whole board) and a 30mA RCD downstream (covering additional external circuits in garage).

I’ve had this problem before which makes me think I’m doing something fundamentally wrong but having failed to get a straight answer from my tutor I wanted to throw it out to the real experts. The OSG is very specific but gives little away about preparation for the tests other than removal of bulbs etc.

Would appreciate your thoughts. I’ve seen suggestions on here about linking line and neutral and testing to earth but can’t see how this would provide a comprehensive set of results.

Thanks.

 

[malcolmsanford]

If there are no obviously spurs coming off the ring, is there an outside light at high lever taken off that ring??

Is there an immersion heater taken off it as a spur, I know it shouldn't be but there is a lot in this game that shoudn't be but is.

Anything in the loft, seen a loft light taken as a spur of a ring before as it was closer that the upstair lighting circuit.

If there is a bathroom fan again normally of the lights, but seen them spured off rings as the cable is closer, that goes for shaver sockets as well

So eliminate everything and if that still gives you a low reading then I would disconnect the ring final circuit from the board, that is L N + E. Then carry out a full continuity of ring final circuit test and then you IR test.

 

[telectrix]

spot on answer there. eliminate the obvious then go for the head scratchers

 

[Sparky Ninja]

Have you done any training in testing before?
Can I ask what the readings were that you were getting?
Have you tried testing with a low reading ohm-meter prior?

 

[Deleted member 9648]

Have you completely isolated the N for the circuit you are testing from the N bar?....if not you will get readings through other neutrals connected to that bar....same applies to the live...either disconnect or MCB off.