Zs or (r1+r2) + Ze?

[telectrix]

Correct, if you take your Ze + R1+R2 = say 1ohm , but your measured Zs= 2 ohm , then you should start looking for loose terminals etc.

but surely then, his R1+R2 readings would have been high if there was a poor connection.

 

[ringer]

but surely then, his R1+R2 readings would have been high if there was a poor connection.

That depends on how well he remade the connections after doing his R1+R2

 

[Sintra]

A circuit I tested today for a solar p.v install which was fitted by another company last august, and the r1+r2 for the circuit was 1.08ohms, the cable was 2.5mm at about 8 metres in length. I was asked to record some lost data which included Ze and Zs. My metrel tester came back as a pass with ref to disconnection times (Ze was 0.16, 16a type B MCB) so according to BGB, 2.87ohms is the max Zs to meet disconnection times.

I did wonder why the reading was so high for such a short run of cable, but as it complied, and my job was just to record the data, I left it.

What would other people have done if they were doing a PIR or whatever they call it now? Would that have sent alarms bells ringing, or would you be happy with that?

Thanks Sam

8M of 2.5mm with R1+R2 of 1.08 ohms would make me investigate a bit further.

 

[needasparks]

I did wonder why the reading was so high for such a short run of cable, but as it complied, and my job was just to record the data, I left it.

Rest my case, that is what most do, which makes the testing worthless as the client is not going to know a good reading from a bad.And what are the chances the installation will be checked again ?

 

[PC Electrics]

If you are going to be taking your 2392, you'd best be fully understanding the correct terminology and the difference between r1, R1, r2 and R2. If you read the chief examiners report for just about any 2391 you will see that one of the prime causes of failure is incorrect terminology.

Thus:
Zs = (R1+R2) + Ze

r1, r2 and rn are the end to end resistances of an RFC.

 

[sambotc]

Unfortunately it is one of this cases of knowing for next time. Like I said before I am happy to hold my hands up and say I wasn't sure, this is the precise reason I asked the questions I have today. In my defense (and genuinely not making excuses) I was only asked to provide data, I dis not install or sign off this job and do not intend too either. This will be passed back to the company for them to decide on what course of action to take and whos name it will be under etc. I do feel though that I am kind of copping out of the situation now in hindsight even though it was within the limits of my test, a poor connection could manifest into a problem in the future.

I will walk away from this scenario having learnt a lot though and with the questions answered I can now make sense of it all. Presumably everybody makes mistakes at some point in their working life especially at the beginning of.
With regards to the terminology, again I appreciate you pointing it out as that will now stick compared to reading the bgb over and over and not actually realizing the difference.

Appreciate the replies

Sam

 

[needasparks]

That was not a dig Sam, just the common theme with testing.
I was talking about just this with my inspector a few months back. He said you would be shocked by the amount of rubbish readings people put down on forms he sees, which stand out like a sore thumb and most of the jobs he looks at never have a reading anything like what was recorded.

 

[sambotc]

No Worries needaspark, I see what your saying and can believe that after going through the training process, which seems to leave a lot of unanswered questions. That's why since qualifying I have been on here reading and asking questions, which I was quite prepared to get some stick for off someone or another, and I can see how it is justified.
Someone like me has the tickets to do the job but without the technical experience and knowledge, thats going to get the backs up of time served sparks and I don't blame them. Just like if the tables were turned and people were asking numpty questions about plumbing/heating/gas/oil etc and held the tickets do work on it, I would be thinking the same!?

I would like to hope that my reasoning for asking questions is more a way for me to learn how to do it right, rather than running around cocking things up and waving the tickets around like they are a degree or something. I'm keen to learn and like to know the correct procedures even if it takes longer etc.

Then we could get on to BS7671 which is a can of worms, and not the best format to learn from. Such a shame they don't produce a more down to earth guide (on-site is closer but still a bit hit and miss) rather than rely on the readers assumption of what the paragraph means etc etc It's no wonder you see time served sparks arguing it out on here when the reg's themselves are clear as mud!

Thanks for the help and no doubt i'll have other questions to ask in the future if you can bear it!

Sam

 

[PC Electrics]

Never stop asking questions Sam. Every day is a school day.

 

[jase]

If you have done all your dead tests then you have measured R1+R2 then when it comes to live tests you measure your Ze, therefore there is nothing calculated about adding the two together as you have Measured both to obtain Zs

 

[needasparks]

Never stop asking questions Sam. Every day is a school day.

Ditto.

 

[ringer]

If you have done all your dead tests then you have measured R1+R2 then when it comes to live tests you measure your Ze, therefore there is nothing calculated about adding the two together as you have Measured both to obtain Zs

Not quite, Jase, because your measurements haven't taken account of the effect that earth bonding has on reducing the resistance of the circuit(s) to earth. Calculated values (yes, even if you measure Ze and measure R1+R2 you are still calculating because you are adding them together) will always therefore (assuming that bonding actually exists!) be higher than actually measured Zs values.