Zs, R1+R2 Measured Values?

[TPES]

When testing its always been said that its important to take your Continuity reading (R1+R2) and that Zs can be calculated (avoiding working live)

So how many of you do all your correct R1+R2 values and then claculate there Zs and put this figure down on your test sheets?

Or are most of you doing both test all of the time on every install you do and recording what you get on the cert?
I know measured values will usually be slightly different to calculated values due to parralel earth paths etc..

If i take R1+R2 reading and then take my Zs reading from the same point on the circuit they very rarely calculate out write using my Ze.

Todays example..

Lighting pendant.

Ze = 0.22
R1+R2 = 1.94
Zs = 0.82

 

[IQ Electrical]

When testing its always been said that its important to take your Continuity reading (R1+R2) and that Zs can be calculated (avoiding working live)

So how many of you do all your correct R1+R2 values and then claculate there Zs and put this figure down on your test sheets?

Or are most of you doing both test all of the time on every install you do and recording what you get on the cert?
I know measured values will usually be slightly different to calculated values due to parralel earth paths etc..

If i take R1+R2 reading and then take my Zs reading from the same point on the circuit they very rarely calculate out write using my Ze.

Todays example..

Lighting pendant.

Ze = 0.22
R1+R2 = 1.94
Zs = 0.82

Yes, and that was probably a domestic installation, wait until you test a metal clad industrial installation!

You need to prove continuity of protective conductors whether by R1+R2 or just an R2 wander lead test.

For the record, I'd rather take a full current Zs measurement than calculate unless there was a very good reason!

 

[Guest123]

Measure every time.

The whole 'avoid live working' from the NIC is a total cop out and an insult to those of us who have been trained to work with electricity competantly.

The latest edition of PE had an article on this topic which was very interesting.

 

[TPES]

This is 48 way Metal clad Invicta3 DB.

I assume this is why im getting odd measure values.

So if you get both these measured values that dont correspond to your Ze at all do you just record both these results...?

No calculating anything, forget the Ze formulas exist?

 

[Guest123]

Your Ze is your Ze regardless

Zs is measured under normal service conditions with any and all parallel paths included as they would be when the installation is in use anyway.

 

[telectrix]

IMO, the calculatged value tells you what you will get theoretically. the measured value will usually be less due to parallel paths, but sometimes is higher due to MCB or RCD affecting the measured value. i had one last week where Ze was 0.16, and Zs measured at the load side of the RCD was 341

 

[chiplard]

If you measure Zs and you are reading through parallel paths then you are getting a value that will obviously be lower than calculation.

I have always calculated Zs so as to get the highest value, to ensure that the circuit impedance concerned does meet the max value for the individual protective device.

Maybe not necessary but I've always done it that way.

 

[roukel01]

I do most of my testing from the van!!!!!! lol

 

[telectrix]

I do most of my testing from the van!!!!!! lol

so it's you that's got the new prototype wi-fi megger MFT.

 

[TPES]

Just want to get my head straight on this one, slightly off topic

I have a type C 10a 30ma RCBO.
Zs = 2.21

With correction factor my max allowed for 10a type C is 1.84

But as 30ma RCD protection this gives me a max of 1667.. meaning my 2.21 is ok... that right?

 

[IQ Electrical]

Yes, it's a standard 2391-10 answer for a circuit with exceeded Zs.

 

[TPES]

i know i know, just testing you IQ, well done... you got it right.. ;o)

 

[TPES]

I doubted my self because on my elecsa chart for max loop values it clearly states these values are also for 61009 RCBO's...

Why would it say that if the RCBO's have 30ma protection meaning your max value will always be 1667 ohms... and not the values on the chart

Thats why i doubted myself at work...

 

[chiplard]

Hypothetical question here

If you did measure the Zs and it was within tolerance and later one of the parallel paths was removed i.e. An extension was built and the incoming water pipe was changed to plastic, then how would you ensure that the Zs value was still within tolerence.

I know that at the time of install it was ok and you cant foresee every future change but surely it is better to ensure that the individual circuit Zs ( by calculation ) is within tolerance.

 

[IQ Electrical]

They're listing the overcurrent part of the rcbo but you aren't relying on that to meet your disconnection time.