Zs or (r1+r2) + Ze?

[sambotc]

A bit confused as to what the prefered method of testing and recording Zs should be?

I normally do (r1+r2) +Ze and record all the details individually, but Zs measured from the furthest point of the circuit is always lower than the calculated, due to parallel paths etc I guess? Should I record the calculated, or measured? Logically thinking I would say measured, as that is true to the installation method, but then why bother learning the (r1+r2) + Ze at all?

Secondly, When I do Ze, if its tight for space getting the main earth out of the board, is it acceptable to remove from service head end and measure off this terminal?

Finally, back to Zs, If there is an additional DB off some henleys from the tails, and I measure Ze back at the true source (probably at the henleys i guess?), and r1 + r2 of the circuit, how can I include any calculation for resistance through the henleys and tails? Test Zs from furthest point? If so, surely I will get a slightly differant set of results due to parellel paths again (main bonding etc?)

Is Ze the only time you need to eliminate parallel paths, any other test it is OK to include them in the result?


Hope that all makes sense

Thanks in advance Sam

 

[1shortcircuit]

What is the wording used on your form that you have to fill in? I believe some ask for measured whilst others state calculated?

I always record the measured:thumbsup

 

[sambotc]

The forms at the moment are supplied from another company, so it literally just says r1+r2 and Zs!

I'm starting to think the calculated method is just theoretical nonsense, and so far can't think of a reason to ever use it now?

 

[needasparks]

Think you have to ask yourself when would you need to know r1+r2 and Zs.
Your testing for a reason not just to collect numbers like a train spotter.
How will you know if a circuit is within contraints if you do not look up your results on your findings.

 

[sambotc]

I see where you are coming from, my Metrel MFT automatically gives me a pass/fail for disconnection times of the protective device on circuit if that's what you are refering too, so maybe its making me lazy? I won't deny i'm new to the testing side, hopefully the 2392-10 course i'm booked on to in 3 weeks will help make these issues a little more transparent!

 

[yellowvanman]

I normally do (r1+r2) +Ze and record all the details individually, but Zs measured from the furthest point of the circuit is always lower than the calculated, due to parallel paths etc I guess? Should I record the calculated, or measured? Logically thinking I would say measured, as that is true to the installation method, but then why bother learning the (r1+r2) + Ze at all?

You can record either - as long as the result is less than the maximum allowable for the protective device.

Secondly, When I do Ze, if its tight for space getting the main earth out of the board, is it acceptable to remove from service head end and measure off this terminal?

Can't see a problem with that, although I would do a Zs/Ipf measurement at the DB afterwards to make sure good connections are maintained.

Finally, back to Zs, If there is an additional DB off some henleys from the tails, and I measure Ze back at the true source (probably at the henleys i guess?), and r1 + r2 of the circuit, how can I include any calculation for resistance through the henleys and tails? Test Zs from furthest point? If so, surely I will get a slightly differant set of results due to parellel paths again (main bonding etc?)

The resistance from your henleys and the DB will be negligible (as long as they are close by), and your meter probably wouldn't be able to measure the difference of Zs at the henley or at the DB!

Is Ze the only time you need to eliminate parallel paths, any other test it is OK to include them in the result?

Only time!

 

[nickblake]

method for testing Ze switch off board , if you can disconnect at the DNO cut out test with earth connected to the cut out block or cable sheet depending on the supply earth ,at the live terminals of the isolator , it also shows that the earth block on the side of the cut out is actually connected , how many earth wires i have have seen connected to these assuming they are connected and they are not , once completed reconnect the earth ,as for the Zs always being lower it does occasionally not very often though I carry out an r1+r2 test and calculate the Zs then do a Zs at the furthest position by doing the r1+r2 + Ze = Zs will give you a rough idea of what to expect when you do the Zs well its the method i use

 

[sambotc]

Thanks for the replies, kind of verified that my way of thinking is similar to those more experienced in testing. I have always been testing Zs as well as the calculated method, which is why I started getting a bit confused over the results. I would say 9/10 the results differ slightly, but that can relate to quite a big differance in PFC. Which of the 2 would you use then, higher or lower?

Other than the refered disconnection times for the protective device, is there a published resistive limit to r1 + r2 anywhere, or is it a bit of common sense and experience that would lead you to investigate a reading over a certain amount?

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

 

[needasparks]

I think if you remember the testing is to verify the calculating is to see if the reading you obtain is correct. You are i see aware that you can get parallel paths so again you can discount some strange readings. Its the un-expected ones that you look for to highlight a possible problem. There are too many people out there that just fill in the forms and not really understanding the results they are getting. After doing a proper testing course i think you may understand but calculating a result yes is worthless on its own and its only when you compare it to the real thing will you get the full picture.

 

[sambotc]

Thats kind of put it into context I think, thanks

Zs being the test and the result recorded, but r1+r2 and Ze, which all need to be measured regardless, can be quickly added together to verify that the Zs test was accurate and also indicate, if a problem does exist, where the fault lies in the installation?

 

[telectrix]

well first get your terminology right!!!!! r1, r2, and rN refer to the end/end resistance of a RFC.

 

[needasparks]

Glad to help fella. I find it quite interesting when i can get my teeth into a good abnomality when testing. Just call me sad

 

[needasparks]

well first get your terminology right!!!!! r1, r2, and rN refer to the end/end resistance of a RFC.

Oh dear, someones woke him up.

 

[sambotc]

well first get your terminology right!!!!! r1, r2, and rN refer to the end/end resistance of a RFC.

soRRy

 

[Marnik]

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.

 

[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.