zs testing

[Jake1579]

Im on an electrical installation course in college and currently on a 3 month trial as an apprentice so thats why im asking zs testing from what i understand in college they teach it you as zs=ze + r1/r2 which i understand but when i go and actually look at zs test myself say on youtube they talk about like for example a ligthing circuit to test it at the switch line and the earth on the celing rose or light switch depending if its awkward to get to but they say test at the last lighting point so my questions are
1. why do you need to test at the last point?
2. does this test take in to account the ze and r1/2?

 

[Engineer54]

Trev, it's very much true that live Zs testing is discouraged, in principle. I have recently been back to college to finally get round to my 2394/5 and have encountered this in practical exams.
The reasoning given is that:

• The EAWR forbids live working unless it is absolutely necessary
• BS7671 allows Zs to be determined by other methods than live testing
• The values used to calculate Zs are both measured values so a meaningful result is obtained

That said, PFC is still expected to be measured directly, go figure !

And PEFC is a test meter calculated value, based on the live test results!! You generally only need to calculate Zs values during the design stage, Direct measurement is the only test method i'd be willing to accept before handing over an installation as being fit for purpose.

There is a distinct difference between live working and live testing, they Ain't the same thing!!!

 

[Andy78]

And PEFC is a test meter calculated value, based on the live test results!! You generally only need to calculate Zs values during the design stage, Direct measurement is the only test method i'd be willing to accept before handing over an installation as being fit for purpose.

There is a distinct difference between live working and live testing, they Ain't the same thing!!!

I'm in complete agreement, just saying how things are.
Not sure if the limitations on live testing are a C&G thing or a college/insurance thing, but we were asked to record calculated values as the regs state this as a permissible method. We were however asked to take live Zs readings where we could use the BS1363 socket adapter lead for the MFT, as this didn't involve taking apart any equipment.

I have to say though that we had an excellent tutor for this course, and what we were taught in the sessions was above and beyond the syllabus and had a lot more relation to the real world. It was just the practical assessment that was controlled.

 

[Adam W]

I'm in complete agreement, just saying how things are.
Not sure if the limitations on live testing are a C&G thing or a college/insurance thing, but we were asked to record calculated values as the regs state this as a permissible method. We were however asked to take live Zs readings where we could use the BS1363 socket adapter lead for the MFT, as this didn't involve taking apart any equipment.

I have to say though that we had an excellent tutor for this course, and what we were taught in the sessions was above and beyond the syllabus and had a lot more relation to the real world. It was just the practical assessment that was controlled.

When I did my 2391 we were taught to measure Zs wherever possible because it gives an actual reading instead of a theoretical one. There isn't much danger involved in plugging in an adaptor or poking GS38 probes into barriered terminals, but if you would have to dismantle a 3 phase socket exposing the live terminals, obtaining Zs by calculation would eliminate any risk.

 

[LeeH]

When I did my 2391 we were taught to measure Zs wherever possible because it gives an actual reading instead of a theoretical one. There isn't much danger involved in plugging in an adaptor or poking GS38 probes into barriered terminals, but if you would have to dismantle a 3 phase socket exposing the live terminals, obtaining Zs by calculation would eliminate any risk.

Why is calculating Zs theoretical? You have measured the Ze and R1 R2 using a calibrated meter.

Just do the maths.





(Tin hat on) I know I'm in the minority here....

 

[trev]

Why is calculating Zs theoretical? You have measured the Ze and R1 R2 using a calibrated meter.
Just do the maths.
(Tin hat on) I know I'm in the minority here....

Because the maths takes no account of parallel paths and does not give the true reading.

 

[LeeH]

Are all parallel paths permanent?

 

[Adam W]

Are all parallel paths permanent?

Bonding should be, yes. What other kinds of parallel paths could there be, bearing in mind during testing anything not relevant to the testing should be disconnected?

 

[LeeH]

Bonding should be, yes. What other kinds of parallel paths could there be, bearing in mind during testing anything not relevant to the testing should be disconnected?

It was a rhetorical question.

 

[Adam W]

Why is calculating Zs theoretical? You have measured the Ze and R1 R2 using a calibrated meter.

Just do the maths.





(Tin hat on) I know I'm in the minority here....

An example that springs to mind is an average speed camera. It takes the time you passed on point and compares that with the time you pass a second point assuming you have travelled at the same speed between the two.
It's an indication, but doesn't account for changes of speed between the two points.

Likewise calculating Zs is a good indication and can be used in situations where it may be dangerous to test live, but it's an indication rather than a true measurement.

 

[Andy78]

Is it not an indication that errs on the side of caution though ? If a calculated value complies then a measured value must also ? Testing always looks for and uses outside values and worst case scenarios.
For the record this is just me being devil's advocate. I was always taught to measure. But a calculated value is at worst a valuable comparison to a measurement....

 

[LeeH]

Bonding should be, yes. What other kinds of parallel paths could there be, bearing in mind during testing anything not relevant to the testing should be disconnected?

You're saying you disconnect parallel paths apart from bonding while taking a measurement? Good luck with that outside of a 2 up 2 down!

A list of parallel paths is endless. Motor sat on a steel platform for example. Try disconnecting that while measuring Zs.

My point is, parallel paths come and go during the life of the install. The CPC is there to stay so why having a worst case scenario such a bad thing?

For example, man fits motor, motor vibrates, man fits anti vibration feet, parallel path lost, Zs increases. Disconnection time now not achieved.

I'm not saying it's better, just cannot understand some people's resistance to calculate.

 

[Adam W]

You're saying you disconnect parallel paths apart from bonding while taking a measurement? Good luck with that outside of a 2 up 2 down!

A list of parallel paths is endless. Motor sat on a steel platform for example. Try disconnecting that while measuring Zs.

My point is, parallel paths come and go during the life of the install. The CPC is there to stay so why having a worst case scenario such a bad thing?

For example, man fits motor, motor vibrates, man fits anti vibration feet, parallel path lost, Zs increases. Disconnection time now not achieved.

I'm not saying it's better, just cannot understand some people's resistance to calculate.

The original question was about a college question, but if a Zs were likely to be increased the person likely to be increasing it should measure the Zs when he alters it.
Similar questions have come up before, and the general consensus is you can't account for alterations which may be made.

 

[LeeH]

You can when only the CPC is in the equation.

I know, I'm playing devils advocate.

 

[Adam W]

You can when only the CPC is in the equation.

I know, I'm playing devils advocate.

What if you were using metallic containment as the CPC?

 

[Engineer54]

You're saying you disconnect parallel paths apart from bonding while taking a measurement? Good luck with that outside of a 2 up 2 down!

A list of parallel paths is endless. Motor sat on a steel platform for example. Try disconnecting that while measuring Zs.

My point is, parallel paths come and go during the life of the install. The CPC is there to stay so why having a worst case scenario such a bad thing?

For example, man fits motor, motor vibrates, man fits anti vibration feet, parallel path lost, Zs increases. Disconnection time now not achieved.

I'm not saying it's better, just cannot understand some people's resistance to calculate.

Why arse around calculating when you can directly test/measure the Zs at any given point on the circuit, and get an accurate value into the bargain!!!

 

[Adam W]

Why arse around calculating when you can directly test/measure the Zs at any given point on the circuit, and get an accurate value into the bargain!!!

Because it can be safer, but it seems we're going around in circles now.

 

[Engineer54]

Because it can be safer, but it seems we're going around in circles now.

Listen, ...if you can't safely undertake Ze/Zs tests or any other live testing, then there's something very wrong and shouldn't be in this industry. Live testing is a fundamental necessity for assessing electrical installations, end of!! I'm not going round in circles, i've stated previously i would not accept a calculated value, it's either a directly tested value or Nothing!!...

 

[Adam W]

Listen, ...if you can't safely undertake Ze/Zs tests or any other live testing, then there's something very wrong and shouldn't be in this industry. Live testing is a fundamental necessity for assessing electrical installations, end of!! I'm not going round in circles, i've stated previously i would not accept a calculated value, it's either a directly tested value or Nothing!!...

Depends on the circumstances.

 

[Adam W]

Listen, ...if you can't safely undertake Ze/Zs tests or any other live testing, then there's something very wrong and shouldn't be in this industry. Live testing is a fundamental necessity for assessing electrical installations, end of!! I'm not going round in circles, i've stated previously i would not accept a calculated value, it's either a directly tested value or Nothing!!...

Who said anything about Ze? Why would you not be able to take a live Ze reading? And how else would you take it?

 

[LeeH]

Listen, ...if you can't safely undertake Ze/Zs tests or any other live testing, then there's something very wrong and shouldn't be in this industry.

There IS something very wrong with the industry unfortunately.

What if you were using metallic containment as the CPC?

Measure is the only solution.

Why arse around calculating when you can directly test/measure the Zs at any given point on the circuit, and get an accurate value into the bargain!!!

Because it can take hours to get ISO locks taken off and reinstated.

And you can get accurate results with a brew in your hand rather then an MFT.

 

[Engineer54]

Who said anything about Ze? Why would you not be able to take a live Ze reading? And how else would you take it?

I did!!

Exactly, so why arse around with calculating Zs, if you can test for Ze you can test Zs??

 

[Engineer54]

There IS something very wrong with the industry unfortunately. Agreed!!



Measure is the only solution. Are you having a laugh or what??

Because it can take hours to get ISO locks taken off and reinstated. So how are you going to take dead test values to calculate??

And you can get accurate results with a brew in your hand rather than an MFT.

I'll guarantee that any calculated value you come up with, will not be the same as the live measured value!!
And coupled to this new convoluted Zs measurement, where it seems you need to know the bloody voltage, even less likely... lol!!

 

[shanky887614]

Why is calculating Zs theoretical? You have measured the Ze and R1 R2 using a calibrated meter.

Just do the maths.





(Tin hat on) I know I'm in the minority here....

Because the maths takes no account of parallel paths and does not give the true reading.

Are all parallel paths permanent?

depends, an install we did had that much steel bolted together and that many swa cables that the results were way better than they should have been, because of the design of the building most steel work was directly connected without bonding cables

 

[DaveyD]

Surprised no one has mentioned the inaccuracy of 'no trip' ZS tests yet. I seldom get a measured value lower than a calculated when on 'no trip'.

 

[Engineer54]

Surprised no one has mentioned the inaccuracy of 'no trip' ZS tests yet. I seldom get a measured value lower than a calculated when on 'no trip'.

A valid point that i didn't consider, as we initially temporarily install spare double pole, TP+N breakers/ isolators, only installing the RCD units at time of testing the RCD's. So i'd say 95 % of all our circuits Zs testing is carried out at full current...

 

[Paul.M]

Surprised no one has mentioned the inaccuracy of 'no trip' ZS tests yet. I seldom get a measured value lower than a calculated when on 'no trip'.

Surprised that no one has mention that you can get your Ze "by enquiry" then test for Voltage, dead test R1+R2.

If the Ze = 0.17
R1+R2 = 0.54
V = 237

Only two tests needed lol

Zs = 0.17 + 0.54 = 0.71
PFC = 237 / 0.17 = 1.39kA

All the maths are thrown out of the window soon as you get the MFT out. The test results will never match the maths in the real world and should only be used as a calculated/estimate of results before testing to compare against.

With the things we have these days like socket adapters and Lightmates, no reason not to safely carry out tests.

 

[markhbyc]

Cant really explain it any better or clearer than this.

Firstly my apologies for interrupting a thread. I'm an avid reader of forums but not well versed with any etiquette .
I have particularly sought to seek your advice as I deeply respect your knowledge and understanding of the 2394/5 examinations.
I am undertaking the 2394 written exam in a few weeks and have been just finalising my revision notes. I have made use of the published Chief Examiners Reports which do tend to point out the basic errors students make.
However one note from a report (Aug 2014) is confusing me, and I would be grateful for your thoughts.
To quote from the report :-
"Candidates were asked to explain the expected pattern of test results when carrying out an earth fault loop impedance measurement at each socket-outlet on the ring circuit. Many candidates incorrectly believed that the results would be the same at each socket-outlet. "
Please correct me if wrong but I have been taught that a correctly installed ring circuit will have substantially the same (R1 + R2) value at each socket outlet (within 0.05ohms). The only exception would be where the ring contains a spur , the (R1 +R2) being marginally higher at this point , proportional to the extra cable length.
I have also been taught that the following calculation is used to calculate Zs , Zs = Ze + (R1 + R2). I'm fully aware of ambient temperature and conductor operating temperature considerations for verification but lets keep it simple for now. Ze is a constant, as mentioned (R1 + R2) should be substantially the same for each outlet so my question is why would it not be conceivable to expect substantially the same Zs values ?
Again my apologies for interrupting this thread, I would be really grateful for thoughts .

 

[telectrix]

Firstly my apologies for interrupting a thread. I'm an avid reader of forums but not well versed with any etiquette .
I have particularly sought to seek your advice as I deeply respect your knowledge and understanding of the 2394/5 examinations.
I am undertaking the 2394 written exam in a few weeks and have been just finalising my revision notes. I have made use of the published Chief Examiners Reports which do tend to point out the basic errors students make.
However one note from a report (Aug 2014) is confusing me, and I would be grateful for your thoughts.
To quote from the report :-
"Candidates were asked to explain the expected pattern of test results when carrying out an earth fault loop impedance measurement at each socket-outlet on the ring circuit. Many candidates incorrectly believed that the results would be the same at each socket-outlet. "
Please correct me if wrong but I have been taught that a correctly installed ring circuit will have substantially the same (R1 + R2) value at each socket outlet (within 0.05ohms). The only exception would be where the ring contains a spur , the (R1 +R2) being marginally higher at this point , proportional to the extra cable length.
I have also been taught that the following calculation is used to calculate Zs , Zs = Ze + (R1 + R2). I'm fully aware of ambient temperature and conductor operating temperature considerations for verification but lets keep it simple for now. Ze is a constant, as mentioned (R1 + R2) should be substantially the same for each outlet so my question is why would it not be conceivable to expect substantially the same Zs values ?
Again my apologies for interrupting this thread, I would be really grateful for thoughts .

you're slightly confused here. when cross connecting the RFC legs, you should get R1+R2 at each outlet will be the same. your Zs readings are taken with the RFC legs connected into the supply, so will be dependent on the resistances when in this configuration. i.e, highest reading will be at thew mid point.

 

[Richard Burns]

you're slightly confused here. when cross connecting the RFC legs, you should get R1+R2 at each outlet will be the same. your Zs readings are taken with the RFC legs connected into the supply, so will be dependent on the resistances when in this configuration. i.e, highest reading will be at thew mid point.

Now why couldn't I have thought of that much easier explanation instead of drawing this diagram!

 

[markhbyc]

Thank you for your reply, makes perfect sense . I will bring it to the attention of my tutor as ironically the very same question came up in a mock paper we went through before the Easter break . There were differing views as to the answer. Again, many thanks.