Insulation resistance 0.00megger ohm

[chris_k]

Just to add a bit of fire to the debate. I have in-front of me a sample 2391 paper with the following question-

An insulation resistance test between P N conductors on three individual single-phase circuits revealed the following values

0.00MOhm; 0.08MOhm; >200MOhm

What circuit conditions are indicated by these values? (3 marks)

How would you answer to obtain the full 3 marks?

 

[apprentice87]

Hi All,

And another newbie stab in the dark coming up!!!

0.00MOhm;

There is a "connection" shall we say with a resistance of less than 10,000 ohms between phase and neutral. Quite what the cause is, we do not know. We can merely say that the reading obtained is outside the range of the test instrument... As the resistance could be anything from zero ohms to 10,000 ohms it could be anything from a direct short to a connected load. Further investigation would be needed. However, if the OCPD protecting this circuit did not trip when the circuit was energised, this would be indicative that the problem was a connected load.

0.08MOhm;

There is a "connection" between phase and neutral with a resistance of 80,000 ohms. This is extremely unlikely to be a connected load, as it would have to be a rather small one as it would dissipate only 0.66 watts... Therefore, it is safe to assume that there is a fault between the two conductors.

>200MOhm

This would indicate a healthy circuit.

[Tin helmet on!!]

john...

 

[johninlondon1]

In college on Tuesday (C&G 2391) tried explaining to a student that 0 MOhm on ins resistor set to MegOhm did not mean 0.00 Ohm's but indicates less than parts of a MegOhm, seems that he could not get the idea of it and after more explaining to him I just gave up as he was adamant that if the meter says 0 MOhms then the reading was nil.

 

[Zeespark]

He will become unstuck quite easily then John.

AndyGuinness

 

[kme]

Hi All,And another newbie stab in the dark coming up!!!

0.00MOhm;

There is a "connection" shall we say with a resistance of less than 10,000 ohms between phase and neutral. Quite what the cause is, we do not know. We can merely say that the reading obtained is outside the range of the test instrument... As the resistance could be anything from zero ohms to 10,000 ohms it could be anything from a direct short to a connected load. Further investigation would be needed. However, if the OCPD protecting this circuit did not trip when the circuit was energised, this would be indicative that the problem was a connected load.

I`d like to clarify that this is referring to a PREVIOUSLY energised circuit - anyone who sees these results, then goes on to energise the circuit needs their canvas overcoat back!!

Although, if readings to CPC are clear; I`d do a measurement of the resistance, to gauge the size of load that is probably still connected somewhere (especially on domestic socket outlet circuit )

0.08MOhm;

There is a "connection" between phase and neutral with a resistance of 80,000 ohms. This is extremely unlikely to be a connected load, as it would have to be a rather small one as it would dissipate only 0.66 watts... Therefore, it is safe to assume that there is a fault between the two conductors.

erm......maybe; but it could also be a neon indicator, or small LV power supply - these can often cause such issues.

Generally, though not always, a follow-up test between P+N to CPC would give a better indication - especially if wiring is FTE, it is unlikely that you`d have a high resistance PN fault, yet a clear reading to CPC

>200MOhm

This would indicate a healthy circuit.

[Tin helmet on!!]john...

This would indicate an absence of loads attached to the circuit - if there is a N-E short, or an L-E reading of 0.35Mohms - you could get >200Mohms between L-N; but the circuit is most certainly not "healthy"

 

[chris_k]

Just to add a bit of fire to the debate. I have in-front of me a sample 2391 paper with the following question-

An insulation resistance test between P N conductors on three individual single-phase circuits revealed the following values

0.00MOhm; 0.08MOhm; >200MOhm

What circuit conditions are indicated by these values? (3 marks)

How would you answer to obtain the full 3 marks?

Well, the recommended answers given for the following readings are-

0.00MOhm; - Dead Short

0.08MOhm; - Low insulation fault

>200MOhm - Healthy Circuit

Knowing the City and Guilds lot you will have to have the exact answer that they have on their sheet. If you didnt put the above then marks lost i would have thought.

 

[kme]

Care to scan the sheet, and post it for us to see? Not that I doubt you or anything..............................but that is complete nonsense.

Find me a definition of a "low insulation fault" - or even a "high insulation fault"

If you could do one test to determine a "healthy circuit" - we wouldn`t have so many meters!!!!!

>200Mohms could signify that the fuse in your probe is blown, and the cable is unknown.

Sorry - thats wrong

 

[steptoe]

or you hadnt even plugged your leads in.

 

[chris_k]

Ill try and get it scanned this evening.

 

[unphased]

Its only a 3 mark answer. Dead short, low insulation and healthy circuit it is, in practical exam answers. If you want to put every incy whincy piece of knowledge down then waste time doing it. Chris k's reply seems more reasonable. Don't try to be too clever on a 3 mark answer, 1 mark for each part. If it was a 10 mark then expand the answer.

 

[brav]

Lmfao loving the banter on here, more enjoyment then being a wwfc fan I tell LOL

Nick1986 anytime u have a client/know-it-all over ur shoulder telling u to do this or tht, politely ask him it go away as u need to concentrate on what ur doing. It makes a massive difference when ur on ur own u can work things out step by step instead of when a client is glued to ur back and stressing u out.