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.