Tragic IR fail on EICR

[telectrix]

1MΩ is a convenient figure dreamed up by the IET. It’s not the calculated value.

I’ll give you a scenario:
If you were to test a 50 year old 75HP 433V 3Ph motor, what would you accept for the supply cable minimum IR for L→L and L→E?

is there a RCD on the circuit? :iamwithstupid:

 

[Knobhead]

It’s a rotten question.

I’ll tell you when I get back home.

 

[Knobhead]

is there a RCD on the circuit? :iamwithstupid:

It’s a motor in a plant, no E/L protection what so ever other than OCPD.

Keep out of this tel, let the youngsters have a go.

 

[shanky887614]

1MΩ is a convenient figure dreamed up by the IET. It’s not the calculated value.

I’ll give you a scenario:
If you were to test a 50 year old 75HP 433V 3Ph motor, what would you accept for the supply cable minimum IR for L→L and L→E?

depends on the age of the cable but i would accept a low ir figure as long as it hasnt degraded a lot from last ir test. if last ir test a year ago was say 10Mohm and this year was 1Mohm i would fail it for example

i have no idea tony

 

[telectrix]

It’s a motor in a plant, no E/L protection what so ever other than OCPD.

Keep out of this tel, let the youngsters have a go.

i was being facetious, tony. whoever heard of a RCD on a motor that size. :38:

 

[Knobhead]

This is from a genuine case, a plant with 250+ motors in a damp, dusty and cold environment. The motors ranged from ½ to 800HP.

The calculation is based on a maximum of 1/10000[SUP]th[/SUP] leakage of the circuit FLC.

2 x the Ph→Ph or Ph→E DC voltage is used for the tests.
For 433V you would use 1000V DC Ph→Ph or 500V Ph→E.

I said it was a rotten question, you need the FLC from the scant information I gave.

You need to calculate the motor FLC taking in to account efficiency and power factor. It’s an old motor so 92% efficiency could be a start 0.87 PF maybe?

But we’ll base this on 75HP 92% Ef 0.87 PF where 1 HP = 764W
There could be another way though.

Anyone care to carry it on?

 

[davesparks]

There are a couple of different calculations which have been used over the years.
For complete installations there was a calculation based on the number of circuits in the installation, I'll look it up when I get home later.
The other method I know is the one Tony posted above using 1/10000th FLC as the maximum leakage.

 

[Knobhead]

It is Saturday night so I’ll leave this until tomorrow evening.

As Dave said it’s an old method but in a large installation it’s invaluable.

I said earlier, 1MΩ was a global figure adopted by the IET for ease, it has no real foundation.

 

[sparksburnout]

No, no, no - job was in London :smile5:
Off topic, but I got asked about an outside light in Chester a while back; I said I could do it but I wasn't gas safe.

Not safe at all if you ask me.

 

[davesparks]

Here we go, IR testing according to the 13th edition.

 

[davesparks]

Found an bit from the 10th edition too

 

[Knobhead]

The 1/10000[SUP]th[/SUP] FLC is still relevant today on large power circuits. They would fail if the 1MΩ recommendation was applied to many installations.

Almost all of the UK’s LV distribution system would be a miserable fail but its still working.

There’s also the minor problem of just pressing the button and taking the first reading that is shown. A long cable needs a 30 second test at least, if the needle keeps rising keep your finger on the button. Every cable has a capacitive element, it needs to charge before a stable reading can be taken.

We tested a 1.7KV cable on a regular basis. At first the reports were coming back at around 5MΩ which would be in the words of the OP a tragic fail. A 5 minute test came back with 50GΩ (50,000,000,000Ω). The meter could go up to 10TΩ (10,000,000,000,000Ω).
The rule I set was if the test reached 1GΩ in 1 minute then the test was a pass, to extend the test leads to stress on the cable.

The one value for all doesn’t take different voltages in to account. 1MΩ, doesn’t fit them all.