Why is Zs higher than Ze+R1+R2?

[alanl]

I've had 2 instances in the last week where the Zs measured result was higher than I would have expected.
Firstly, a brand new shower feed, R1+R2 of .11ohms, *Ze (measured) of .31.
I would expect a measured Zs of between .35 and .45 for this, yet it was .55ohms.

Then yesterday, connected a new boiler to an old feed. 2 metres of cable, gave a R1+R2 reading of .05ohms. TN-C-S, I didnt measure Ze, but in a suburban village, so unlikely to be above .35ohms. Anyway, measured Zs was .58ohms.

Clearly both are well within limits, but I cannot see why the measured Zs is higher than a calculated reading, as I would expect the measured one to be lower due to parallel earth paths.

Any thoughts?
Thanks
Alan.

 

[MarkieSparkie]

Hi Alan, This is apparent anomaly usually encountered when relying on one or more no-trip loop impedance measurements (usually the Zs for obvious reasons), particularly if the environment is electrically noisy when making the measurements. If you have made a normal high current Ze measurement and R1+R2, Zs=Ze+(R1+R2) so there is really no point in taking a Zs measurement just do the calculation. I await the brick-bats for suggesting this because a lot of sparks are of the opinion that Zs is the most important measurement for reliability and confidence. I'm sympathetic to this view but suggest it's only of real value if done at high current.
The virtually all apprentices I've encountered in recent years try to make all loop measurements in no-trip and blindly accept the readings regardless.:dunce2:

 

[alanl]

Thankyou, so it is an error by introducing the 'no-trip' function when measuring. That makes sense.
Ta
Alan.

 

[telectrix]

what you have to remember is that both your (R1+R2) and your Ze measurements are taken without consideration of A. main isolator. B. RCD and MCB. whereas the Zsreadings are taken with these devices included in the circuit atr the time of measuring. These devices can introduce a variation in the expected reading due to their internal resistance/components.

 

[Amp David]

One example I have encountered is a 0.45 difference between the incomming and outgoing side of the RCD in a dual RCD board, when testing Zs

 

[telectrix]

One example I have encountered is a 0.45 difference between the incomming and outgoing side of the RCD in a dual RCD board, when testing Zs[/QUOTE

that's nothing the other week i got 0.22 on the incoming side of an RCD and 349.5 on the outgoing.and

where's paul m now i'm all suitede up coming to get him for that last comment.

 

[Amp David]

Bit of a difference there then!

 

[telectrix]

Bit of a difference there then!

just a bit.was a MEM RCD, 16th ED. board with RCD as main switch. tight bar steward wouln't pay for an upgrade so i just tested the RCD. problem came up when boiloer service tech. checked Zs as part of his tests on the boiler. his comment on the service sheet was " unable to connect supply to boiler as EFLI is unacceptable at 351ohms". underneath i wrote in red " max. permitted Zs is 1667ohms!"

 

[Deleted member 9648]

Hi Alan, This is apparent anomaly usually encountered when relying on one or more no-trip loop impedance measurements (usually the Zs for obvious reasons), particularly if the environment is electrically noisy when making the measurements. If you have made a normal high current Ze measurement and R1+R2, Zs=Ze+(R1+R2) so there is really no point in taking a Zs measurement just do the calculation. I await the brick-bats for suggesting this because a lot of sparks are of the opinion that Zs is the most important measurement for reliability and confidence. I'm sympathetic to this view but suggest it's only of real value if done at high current.
The virtually all apprentices I've encountered in recent years try to make all loop measurements in no-trip and blindly accept the readings regardless.:dunce2:

I've been griping about this for ages,and even sent a new megger meter back recently as 5 tests on the same point would produce 5 wildly different readings.....I was informed this is 'normal'....and was asked if I needed to be shown how to carry out testing:evil:

IMO manufacturers have to sort out the no trip test,it is unreliable so whats the point in it?

 

[Amp David]

I've been griping about this for ages,and even sent a new megger meter back recently as 5 tests on the same point would produce 5 wildly different readings.....I was informed this is 'normal'....and was asked if I needed to be shown how to carry out testing:evil:

IMO manufacturers have to sort out the no trip test,it is unreliable so whats the point in it?

Remember now you mention it. I did ring Megger to ask the question regarding the higher no trip results. His response was basically, 'do a few tests at different points on the circuit', ie. up stream and down stream of the RCD and if you get a difference this is why, its the RCD causing it.

Not exactly helpful as this we already know, was looking more for a cure to it.

Its one reason I am wary of spending my hard earned on a new meter TBH, no just a new Megger but other brands

 

[Paul.M]

and where's paul m now i'm all suitede up coming to get him for that last comment.

Here I am! I was going to say that post 4 that you did was the best explanation of this problem but I won't now if your going to be like that. Erm think I just did

 

[Amp David]

^^^^ I never quoted that ???

 

[Paul.M]

^^^^ I never quoted that ???

Sorry mate I didn't realise that I messed up on the quote name, I'll take it off.

 

[Amp David]

Thought i was going mad then!