meggar 1552 zs test

[Sparky Ninja]

I'll email megger for you now and see what they come back to me with.

 

[Sparky Ninja]

Ok, here's what I asked:

Hello there,

I have a megger MFT 1552 tester and have an enquiry to the non-trip Zs test.
I understand that a higher result on the non-trip test, when compared to the standard LOOP test can be obtained.

What I would like to know is:
a) Why this is (I'm thinking something to do with applied test currents)?
b) What level of inaccuracy is deemed acceptable by yourselves? This is so that I can monitor the inaccuracy when self checking and understand when to get them checked out if necessary.

Many thanks

their response...

Hi David,

The MFT uses a 15mA test current live to earth on the no-trip range with an accuracy of +/- 5% +/- 0.03ohms. It also has +/- noise margin due to the low current. The hi current range has the accuracy +/-5% +/-0.03ohm. So it is the noise margin that causes the difference in reading. I would suggest appliances are disconnected from the circuit under test and any heavy loads on the circuit or nearby circuits are switched off to keep the noise down.

Check boxes are a good way of checking the calibration of your testers. I have attached a data sheet for our MTB7671 which you can use to check calibration as it stabilises the loop reading of where the check box is plugged in.


Regards,
Kerry Burdett
Dover Technical Support
Megger Limited Archcliffe Road Dover
Kent CT17 9EN England

So according to this it's the 'noise margin' that is creating the difference. So it might be worth removing loads from circuit as mentioned (I'll be trying this myself at the next opportunity). This means obviously that there is no exact figure for us to keep an eye on, they were kind enough to suggest I buy their self checker though.

I've attached their attachment to this post..


Hope this is helpful.

 

[Sparky Ninja]

Hi Wilder

i am totaly buggered, my initial question to meggar was, why my new tester is giving a higher reading than the ze + R1 +R2 an non trip test of zs.
they suggest it is due to the rcd.
further to this i then ask, is it a case that zs does not allways equal ze +R1 +R2 within reason.
but also the result i put on the cert for zs is not the one i get from the tester but rather the calculation of R1 + R2 + my ze reading.

god help me

Ok well my evaluation of your concern is that you are 'told' Zs = Ze + (R1&R2), yet your tester is clearly saying something else..

My interpretation of this is that both are correct.. you just need to make sure that when you carry out the tests, it's done with the correct procedures in mind.
By design, Zs will equal Ze+(R1&R2), however by measurement it can be either lower, similar or higher.
Lower being with presence of parallel paths (for example conduit containment systems),
Similar will be most likely if it's a brand new installation and you are at the initial verification phase or just doing a completion cert,
Higher if the installation is dated and there can be increased impedance from things like loose, damaged connections. Now with above we have to consider noise on the loop test with loads as well.

Of course a couple of these factors can also be combined on an installation, so what's key is your initial inspection at which time you can assess for yourself what factors can be considered that may affect your measured Zs.

I stick with measurement, then compare it to the calculation. I will always record the measurement unless it's too high. If it's too high some further investigations will normally resolve the issue.

There are many other variables also that can help in your judgement on the acceptable level of Zs..

 

[kevinjstanley]

This interesting wilder, i went around the house turning everything off as i too thought of this being a issue, however the result was still way off what was expected from ze and R1 and cpc.
Also the results from zs were as shown previously way off any 0.03 ohms margin.
this is a brand new tester 5 days old, so to buy their self checker .........we know what we think of that idea

 

[Sparky Ninja]

Have you tried a sample test on a non-RCD circuit regardless of comparing it to the calculation?
Carry out a Loop test, then carry out a non-loop test and see the difference between readings.. with loads removed.

If it's above the margin, what with it being 5 days old, I'd take it back. It's very likely though that the guy who sold it to you will have no idea what you are talking about when you say what's up with it.. I took mine back to the electric centre once and they just swapped it.

 

[nrg]

Just to Add, I have a 1552 and testing a circuit prior to a new board change had a loop on a ring circuit of 0.53.
After swapping the board and carrying out my test, I tested the same socket and got a reading of 1.03ohms. I called megger and after reporting the issue a very knowledgeable gent got back to me and said this seemed to occure when certain Manufacturers RCD's were installed (something to do with the coil size and greater resistance). I swapped the board out again from the Hager board to an MK board just to see if this was correct and sure enough I got a reading on that socket of 0.58 which was near enough for me! Anyone else had this issue ?

 

[Sparky Ninja]

Just to Add, I have a 1552 and testing a circuit prior to a new board change had a loop on a ring circuit of 0.53.
After swapping the board and carrying out my test, I tested the same socket and got a reading of 1.03ohms. I called megger and after reporting the issue a very knowledgeable gent got back to me and said this seemed to occure when certain Manufacturers RCD's were installed (something to do with the coil size and greater resistance). I swapped the board out again from the Hager board to an MK board just to see if this was correct and sure enough I got a reading on that socket of 0.58 which was near enough for me! Anyone else had this issue ?

Yes.
Different devices can create different levels of impedance. This is one of the arguments against the NICEIC's new strategy saying would should only calculate Zs values for circuits where working live would be required in order to take a measurement.

 

[malcolmsanford]

Yes.
Different devices can create different levels of impedance. This is one of the arguments against the NICEIC's new strategy saying would should only calculate Zs values for circuits where working live would be required in order to take a measurement.

Spot on Widdler that was an argument I had with NICEIC a while back when he told a guy I know that Zs should not be done live but by calculation.

I was taught and now do that Zs by measurement is a design concept. On a working installation I want the zs to include the parallel paths, the protection device impedances and anything else that is thrown in, as that is what the installation will be at when a fault occurs, not some calculation on a piece of paper, their answer "Should avoid live testing or work when it is not necessary"

 

[telectrix]

heartily agree with both of you malcolm & widdler. A fault occurring sees the RCD, MCB, and all parallel paths, so Zs should be measured. what could be worrying is when trhe calcoulated value is within the max. allowed, but the measured value is not. will the protective device trip within the specified time. ?