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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 !
 
I do try to keep on top of it but can be hard during the weekdays.
 
1. why do you need to test at the last point?That is where the highest reading will be obtained,
the highest reading is used to compare to maximum values


2. does this test take in to account the ze and r1/2? Zs is the reading obtained which is a value of Ze with R1+R2 adding to it and other parallel earth paths (like services) reducing this impedance

Cant really explain it any better or clearer than this.
 
I feel sorry for the lads learning though, because naturally they are going to follow the advice of their tutors, until they can get on site a be guided by an Electrician.
ye i was quite shocked when i started my trial which im still doing a lot of it is so different than in college but saying that the work i do is mainly industrial at work
 
ye i was quite shocked when i started my trial which im still doing a lot of it is so different than in college but saying that the work i do is mainly industrial at work

I'm also Industrial and I defy anyone to get through their working life without carrying out live tests.
 
Right Jake it looks like your ok to post now so my work is done, I'm off to save some stranded puppies and children now.
 
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!!!
 
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.
 
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.
 
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....
 
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.
 
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.
 
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.
 
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....
 

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