UK Is 0.00 an acceptable R1+R2

[Apex13]

Hi, I am currently installing a DB in a plant room which will supply part of a large workshop.
The DB is supplied from a MCCB panel board which sits right next to it meaning the tails supplying it are barely 3m long.
The tails feeding the DB are 50mm with a 35mm earth.
As with any circuit it requires a R1+R2 test but with the size of the tails and them being so short the reading is too low to even register on the meter so I am just wondering if noting on a test cert a reading of 0.00 is acceptable.

Calculating the theoretical resistance of the R1+R2 come out a little over 0.002 ohms so I’m no surprised it doesn’t register on the meter as even 0.01 but is it better to note the theoretical reading in circumstances like this or stick to what the meter shows and add in the comments on the cert the reason for the reading?
TIA

 

[westward10]

I test a lot of large cables and end up with this so just enter 0.00 as that is what your meter registers.

 

[Apex13]

Thanks, will do.
I would normally have just entered it without another thought but this cert is going to be part of a job that will be part of an NIC-EIC assessment shortly so wanted to make sure it was actually acceptable.
Cheers.

 

[westward10]

They will understand. You can get meters that read lower, normally four wire testers but they don't apply 200ma as required by BS7671.

 

[Brother Blue]

I was taught to enter <0.01 as there is a resistance there, the value cannot really be 0.00.

The tester only displays 0.00 as it has rounded down a value of less than 0.005

 

[westward10]

I agree it can't be zero but that is what your tester displays. I also agree you could quote it as you say. Either way on an assessment the assessor should realise the reason as to the figure you have quoted.

 

[Risteard]

I've always entered it as <0.01 Ohms, as there is obviously a resistance. But that could just be me being a bit ----. Not stating that it's the only way.

 

[Rockingit]

Results can be found by........ measurement, enquiry or calculation.

To my mind, it is better to put the calculated value that you have of 0.002Ω because otherwise any other normally calculated values such as pfc automatically become effectively void as well (230/0 = 0, 230/0.002 = 115,000).

 

[littlespark]

My advice would be to put something in.

The assessor may not have the brain cells to think, and will just be of the impression that “it can’t be zero” Period. Full stop. Computer says no.

 

[Brother Blue]

I agree it can't be zero but that is what your tester displays.

Yes it is what the tester displays, but an electrician should have sufficient technical knowledge to understand why the value cannot be truly zero and act appropriately.

I object strongly to the idea of just writing down what is displayed on the screen and moving on, the person operating the tester should be capable of understanding and interpreting the results of the test.

 

[sunray]

"0.0Ω measured
0.002Ω calculated"

 

[newfutile]

I alway comment on my audits about 0.00 ohm to gas and water bonds.
It's not a superconductor.
I guess your running 50mm cable for line and earth?

 

[7029 dave]

Suppose you could argue the test instrument has been calibrated, with a SN that will be recorded on the test sheet so if the meter shows 0.0Ω measured that should be the entry.

 

[Simon47]

230/0 = 0

No it isn't !
In mathematics, something divided by zero = infinity. Though normally if you do it in software it'll throw a "divide by zero" error as few (if any ?) programming languages support infinity.
Unless the something is itself zero, then it gets really interesting. Zero divided by something = zero; but something divided by zero = infinity; but something divided by itself = one. So the best thing to do is just never divide by zero.

 

[Brother Blue]

Suppose you could argue the test instrument has been calibrated, with a SN that will be recorded on the test sheet so if the meter shows 0.0Ω measured that should be the entry.

No amount of calibration supercedes the laws of physics, 0.00 ohms is impossible under normal conditions and any even vaguely competent electrician will understand this.

 

[westward10]

No amount of calibration supercedes the laws of physics, 0.00 ohms is impossible under normal conditions and any even vaguely competent electrician will understand this.

I think we all understand this so what is the solution. Calculating the value would not be acceptable as it has not been proved by a test. Testers we use day to day do not verify such low resistances and those that can do not deliver a 200ma test current as required by BS7671.

 

[sunray]

I think we all understand this so what is the solution. Calculating the value would not be acceptable as it has not been proved by a test. Testers we use day to day do not verify such low resistances and those that can do not deliver a 200ma test current as required by BS7671.

Write:

"0.0Ω measured
0.002Ω calculated"

 

[westward10]

Write:

Only a test can prove your calculation and it is a schedule of TEST results.

 

[sunray]

Only a test can prove your calculation and it is a schedule of TEST results.

I agree except seeing "0.0" makes one have to look into it, seeing 0.002 as a calculation against it gives the instant solution and additionally shows the author has thought about the apparent confussion.

 

[westward10]

I agree except seeing "0.0" makes one have to look into it, seeing 0.002 as a calculation against it gives the instant solution and additionally shows the author has thought about the apparent confussion.

It is all very well put a calculated figure down but how do you know the installed conductors actually tally with that calculation?

 

[Simon47]

It is all very well put a calculated figure down but how do you know the installed conductors actually tally with that calculation?

Because you've done a measurement and the reading (0.0) tallies with what you'd expect for that scale of resistance (0.002) and the resolution of the instrument.
What you don't know is whether the actual is (say) 0.003 instead of 0.002, but does it matter ?