Zs testing

[Adcrop14]

I ve had to move some sockets and a boiler feed around in a kitchen. I am now testing what I have done but for some reason the RCD is tripping whenever I am testing as, I have checked that the tester is set to 30mA as the red is. I have done insulation resistance between earth and it's hard down. Boiler circuit has tested fine. Any ideas?

 

[sparkytim]

Hi

Does your Loop tester have a NO trip range?

If not a normal Zs test at 25 Amps will trip a 30 m A RCD

 

[Zeespark]

What tester are you using?

AndyGuinness

 

[steptoe]

I have done insulation resistance between earth and it's hard down

what does this translate to?

 

[Manator]

An old term to say its really good

 

[steptoe]

I did wonder, I had it the other way round, usually if I talk of something being down its at fault.

I wonder is it actually an RCD or RCBO, sometimes RCBOs will still trip, esp the smaller sizes.

 

[kerching]

I did wonder, I had it the other way round, usually if I talk of something being down its at fault.I wonder is it actually an RCD or RCBO, sometimes RCBOs will still trip, esp the smaller sizes.

Agreed! Back in the day with the hand w4nked megger infinity was to the right and 0.0 to the left. So an earth fault would be 'down' and a good reading would be 'clear' or 'high'

Now its just freaking numbers flashing around on a digital display............... :coat

 

[electrician786]

Try putting the mentioned circuits on the non RCD side of the consumer unit while you conduct the Zs tests. Then connect back after tests. Remember NICEIC recommend calculating Zs due to the hazard of working live. It's ok if you do not have to take any electrical accessory off the wall like a socket outlet, because you can just plug your tester into the socket outlet without taking the front off.

 

[Manator]

Steptoe I was indeed taking the proverbial, the op stated that he had a fault and I was in no mind to humour him.

Electrician786, you may find that whilst the NICEIC did indeed recommend calculations of Zs, this is to an extent made invalid as an option due to new testing methods, and individual risk assessments. The main arguments for calculation of Zs was down to the testing methods used for lighting circuits only, working at heights whilst trying to get individual probes on three seperate terminals, but with the advent of items like the kewtech lighting test modules, it is now possible to do this test in complete safety. What you can do is either test, or calculate, both are still acceptable on your EIC.

 

[steptoe]

Try putting the mentioned circuits on the non RCD side of the consumer unit while you conduct the Zs tests. Then connect back after tests. Remember NICEIC recommend calculating Zs due to the hazard of working live. It's ok if you do not have to take any electrical accessory off the wall like a socket outlet, because you can just plug your tester into the socket outlet without taking the front off.

you cant do that,

you will have altered the circuit after your test, so it needs tested again!

and you think Im rough!!!

 

[electrician786]

If you read my complete post I did tell him he should calculate the reading instead.

Manator I was told on my 2391 practical exam to calculate Zs value. The lecturer was quite adamant for me not to take any accessory fronts off, due to the dangers, and also the risk of loosening the connections.

 

[Manator]

If you read my complete post I did tell him he should calculate the reading instead.Manator I was told on my 2391 practical exam to calculate Zs value. The lecturer was quite adamant for me not to take any accessory fronts off, due to the dangers, and also the risk of loosening the connections.

Doing an EICR is totaly different and for practical reasons it is advised to calculate Zs. However some people assume that actual readings should never be taken even on a new install. Zs readings are an important part of the testing procedure, as it verifies and confirms your previous readings, and is one of my pet hates when I see new installations that have been filled in using Zs=Ze+(R1+R2).

 

[apprentice87]

Hi Manator and All,

You say;

"Zs readings are an important part of the testing procedure, as it verifies and confirms your previous readings, and is one of my pet hates when I see new installations that have been filled in using Zs=Ze+(R1+R2)"

Exactly.... Testing Zs, although there can be problems from a theoretical point of view sometimes, is basically, the final guarantee that all your calcs are correct. It is the "proof of the pudding" if you like.

Put it like this.... Let us say you are driving your car. You see a speed camera in the distance, what would you rather: look at your speedo and KNOW that you are only doing 30, or decide that the street lamps are 200 feet apart and that you are doing 44 feet a second and time how long it takes between lamp posts and work it out from there!!!!!

When you are stressed and tired, your meter [speedo] will be a lot more accurate than your calculations...

I know that the NICEIC try to say you should calculate Zs, cos it is dangerous to measure, [live working and all that] but they should get a grip.. You might as well say starve yourself to death because you might choke eating.....

The NICEIC only say this to protect themselves, they would rather someone get electrocuted because your calcs were wrong and YOU get the blame, than say "yes, go and test Zs" and then risk someone saying "Hello NICEIC, i got hurt testing Zs, you told me to do it, i am suing you" that is the truth of the matter.

It is a bit like the welding institute telling welders to use glue, "because you might burn yourself with that nasty hot welding lark!!!!!!!!!!"

John......

 

[Technician]

Manator, I have read your post with interest.

In the last line your pet hate is Zs = Ze + (R1 + R2).

I take it then that it is your pet hate when this is

the only calculation for Zs and NO live reads have

been taken?

Apologies for being a bit picky.

 

[Ali]

apprentice87 you are, as usual, the voice of reason.

 

[S60TEM]

I always test Zs where it is safe and practical to do so. Aslong as your safe on GOOD step ladders, not stretching so as to unbalance yourself and are using probes to GS38 then testing at ceiling roses is possible also....IN MY OPINION (before someone jumps down my throat about HASAW or EAWR etc etc)

As said, it's the proof of the pudding that there is an earth present and if also 'helps' you identify the extremity of the circuit in question.

 

[apprentice87]

Hi all,

A bit of a follow up.....I know that the NICEIC do not like what they would class as "live work" but they are barmy... How can you properly establish what Zs is, unless you know what Ze is, and you will HAVE to test that live..

I will tell you a little story.... When planning these things, it might be better to play on the "safe side" as it were, and just say you arrived at a figure for Ze "by enquiry" and add R1 + R2 to that, as, then, at least you know, that if over the years Ze creeps up due to deterioration of the DNO installation, that, at least you are covered...

The trouble is,

Firstly, if you did that, you would not be able to meet disconnection times [in theory] in a lot of the jobs you do, [as your "theoretical" Ze would be so high to start with] and;

Secondly; "at least you are covered" but are you?????

Well, no, definitely not.... It would be nice to be able to say that "well, i know that R1+R2 are low enough in "my bit", and so, if the DNO allow Ze to go over the limits, then they are responsible.

Trouble is, they are not limits.. All this stuff you read about TNS .8 ohm and TNCS .35 ohm are only guidelines, illustrations if you like. The actual figure can be higher, and the DNO's freely point this out, so you MUST get the figure by testing.. The only figure i think that they HAVE to observe, is the Ra for their rod at the transformer end of the supply [21 Ohms]

Anyway, back to the little story i was going to tell you.. About a year ago i had a new supply installed to my shed. [3 phase TNCS] Firstly, they turned up with the wrong size cable, [but they would not admit it] and it was only after i said "look, that is the cable size on the quotation, the planners do not arrive at cable sizes by guessing, i do not care what is on your job sheet" that they went and got the right size. Once they got the right cable they discovered the "new" bit was 2 feet too short.....

Anyway, they got it all installed and off they went. A few months later, having bought my new MFT, i had a bit of a play with it. I thought i would test Ze on my new supply. One phase was .52!!!!!! on TNCS!!! I did not know what do think.... My main concern was that as this was obviously a problem with one conductor in the service cable, and that as i loaded it up, there would be more and more of a divergence in phase voltages, and that it would damage my motors. What was worse, was that purely by chance, the faulty phase was L2, the same phase as the house is on, but when i measured in the house, i got a sensible reading...

One day, a few days later, the DNO were working only a few hundred yards from where i live. I downloaded my figures for both the house and the shed, printed them out, and went and showed the DNO people. They came straight round, and guess what.....The cable jointers had forgotten to tighten the screws in the cut out!!!!! You could not make it up!!!

Sooooooooo, never ever rely on the figures the DNO give you, ALWAYS test, and if you are going to test Ze, then you might as well test Zs !!!!!!

john..

 

[Technician]

"TNS .8 ohm and TNCS .35 ohm are only guidelines"

AND Maximum Values. There is a world of difference

between live testing WITH live parts exposed and then

live testing WITHOUT live parts exposed.

Interesting post Apprentice 87.

 

[steptoe]

Where do you get those as max values technician?

 

[Technician]

OSG after the page where they are stated.

As you were, page 11.

"1v typical maximum values for earth fault loop impedance

for TN.. etc."

HTH Steps.