Measuring Zs with all circuits connected or just the circuit being tested.

[martygturner]

Good morning,

If we test a circuit with all circuits live we get a substantially higher Zs reading than when only having that circuit being tested live.

Eg. a small ish radial with 4 sockets protected by a 16amp MCB with a 30ma RCB gives a R1+R2 reading of 0.43 and a calculated Zs of 0.74 (Ze is at .30), live tested with all circuits energised we get a reading of 1.02 on no trip setting and when only that circuit is live we get 0.80

Cable length is line with the R1/R2 - 22m measured ( we could trace that cable all the way from final to the CU.

There are no parallel paths to earth, all services are in plastic.

So whilst the circuit is fine at either Zs figure the question comes down to which Zs figure is correct and should be noted.

 

[securespark]

Don't know about other testers but when I use my Alphatek (on an non RCD circuit) on a no trip (soft test) setting it always gives a higher reading.

I would always note the higher reading.

Of course, the problem comes when the reading is too high, in which case investigation is required.

Test equipment can sometimes pick up inaccurate readings.

I had an RCD to test which was failing the half times test. But with all circuitry disconnected, in isolation, it tested fine.
It was appliances that were plugged in that were the problem.
This is why RCDs should always be tested in isolation.

 

[Jurassicspark]

When you were testing with all circuits energized, you're likely seeing a higher Zs due to voltage drop in the circuit especially if there's any connection between neutral conductors in the board, or any other shared paths. Just get record the highest reading and see if it is within spec if not then you'll have to do it another way.

 

[ericmark]

What I think a loop impedance test does, is measure voltage, then put in a load, then remeasure voltage, should anything cause the voltage to change during the test, there will be an error with the results. I note test twice and always slightly different results.

As to RCD ½ test, as long as you put a clamp-meter on the tails,

and you know the back-ground leakage, then the ½ test should be OK, in early days of RCD's I found the tightening of the cable clamps could affect the tripping point, so it is important to test without touching the terminals after the test.

Today the electronics mean that seems less of a problem, but you are testing existing, so don't touch the terminals after testing. Let's face it, a new RCD should not need testing, it should have been tested before leaving the factory, we don't test MCB's, so the whole reason to test on fitting is slight warping of the case, can stop it working within spec.

Today, with a ramp test we can add the back-ground leakage, so no need to disconnect to test.

 

[JohnW2]

Eg. a small ish radial with 4 sockets protected by a 16amp MCB with a 30ma RCB gives a R1+R2 reading of 0.43 and a calculated Zs of 0.74 (Ze is at .30), live tested with all circuits energised we get a reading of 1.02 on no trip setting and when only that circuit is live we get 0.80

Were the other circuits significantly loaded when energised?

 

[martygturner]

Thought I would finish this, after swapping out the RCD for RCBO's all the measured Zs are now close to the calculated as makes no difference, ie within 0.01 to 0.06.

It seems as if the original RCD was adding something to the party even though it passed all its tests.

 

[Murdochcat]

It seems as if the original RCD was adding something to the party even though it passed all its tests.

A very common occurrence.

The difference you noted isn’t really an issue as it’s well below the maximum Zs but if you were close to or over the max Zs then it needs looking at.

If you ever look at an EIC or EICR when the Ze + (R1 + R2) = Zs consistently you know some of the figures have been fabricated

 

[ebee]

My meter measures Zs on no trip and it`s resolution is 1 ohm.

Therefore a little bit disappointing - it is my fault, I did not read the spec properly before I bought it. That will teach me!

However I look at it from this point of view - I have got the Ze (or Zdb) figure from testing and also the R1 + R2 so the Zs non trip test is only a confirmation that Zs exists and is not way out.
I used to note that many of the top brand testers when used on the non trip Zs setting whilst actually testing a non RCD circuit usually gave a figure that was out of alignment with that testers own trip test setting anyhow, by a factor of around two.
Once I put this into perspective I concluded that, despite their better resolution, it was only a slightly better confidence check.
In an ideal world the repeatable readings would align up "within the parish" on multiple tests on both settings.
That was around 2006 to 2010, how much things have changed since then I have no idea!
Testers using D-Lok also gave these differing results.

 

[JohnW2]

My meter measures Zs on no trip and it`s resolution is 1 ohm. .... Therefore a little bit disappointing - it is my fault, I did not read the spec properly before I bought it. That will teach me!

I'm not sure that you should blame yourself all that much ... it makes liuttle sense for someone to produce a meter designed to measure Zs which only has a resolution of 1 Ω !!

However I look at it from this point of view - I have got the Ze (or Zdb) figure from testing and also the R1 + R2 so the Zs non trip test is only a confirmation that Zs exists and is not way out. I used to note that many of the top brand testers when used on the non trip Zs setting whilst actually testing a non RCD circuit usually gave a figure that was out of alignment with that testers own trip test setting anyhow, by a factor of around two.
Once I put this into perspective I concluded that, despite their better resolution, it was only a slightly better confidence check.

I must say that I've always been a bit sceptical about many of the 'low resistance' measurements we do (or, at least, the way we interpret the results of those measurements). We quite often see concerns being raised about Zs figures which are just slightly above the "maximum Zs" to achieve ADS (maybe only 'above maximum' by 0.1 Ω or 0.05 Ω) - yet, as you point out, measurements made in different ways can differ by far more than that.

I must also say that I don't really know how these 'Zs measurements' are done, particularly with TN-C-S supplies, when much of the "Ze path" may already be carrying very high currents.

 

[ebee]

I must say that I've always been a bit sceptical about many of the 'low resistance' measurements we do (or, at least, the way we interpret the results of those measurements). We quite often see concerns being raised about Zs figures which are just slightly above the "maximum Zs" to achieve ADS (maybe only 'above maximum' by 0.1 Ω or 0.05 Ω) - yet, as you point out, measurements made in different ways can differ by far more than that.

Doing the "old fashioned way" and putting bypass links on the RCD gives a decent result on the trip side in accordance with the Zs = Ze + R1 +R2 results obtained with the same meter. It used to be quite a common practice before D-Lok became the rage or if D-Lok failed to block the RCD successfully.
I concluded that all the differing "non trip" testers at the time were usually not accurate in reality therefore the 1 ohm resolution was possibbly a more honest determination

 

[martygturner]

From a developers point of view the frustration comes from circuits that are approaching their Zs limit or are indeed slightly over it when connected to an RCD. Whilst this is episode is in our house we have come across it at various renovations which has added significant costs, ones that at the time we were held to ransom by. Eg if you want to rent out then you need a EICR, you get a report that states that an individual circuit has a C2 due to a high Zs so you pay to get that fixed. Then when you have replaced the perfectly good consumer unit with a new RCBO one the same circuit has now magically become fixed... so is a question of its an inherent design condition of a dual RCB, or is it the testing of that type of installation that creates an issue that is not there ?

Is there any guidance out there that states that Dual RCD boards have significantly higher Zs readings than RCBO ? or is that what the its all ok if the reading is below 1667 Ohms with a rcd... because if it is I have been rinsed a few times.

 

[Murdochcat]

Relying on 1667 ohms is a cop out

A C2 for a “high” Zs is open to debate without a specific example

 

[JohnW2]

Relying on 1667 ohms is a cop out

Not just a cop-out but, in terms of regs, not really allowed (altough I seem to recalll thatthey do say something about "when ADS cannot be achieved by OPDs' - which I don't really understand)

A C2 for a “high” Zs is open to debate without a specific example

Is it (in a TN installation)? With the possible caveat mentioned above, for better or worse the regs require a guarantee of rapid enough disconnection mediated by OPDs, the implication being that they regard it as 'unsafe' to not satisfy that requirement. That being the case, it would seem that many would feel that a C2 was always appropriate?

In terms of the real world, and common sense, rather than 'the regs', it might be a little different, not the least because the regs are prettyu conservative. In particularly the 'maximum Zs figures' assume the worst case of OPD behaviour - e.g. they assume that magnetic tripping of a B-curve MCB/RCBO will require a current of 5 x In, whereas it might well trip with not much more than 3 x In.
Furthermore, even if Zs is slightly too high for magnetic tripping in the device concerned, it will still thermally trip in not much longer time, and the 'required disconnection times' are essentially arbitrary. However, EICRs are largely about 'reguklatory requirements', not common sense!

 

[Risteard]

Don't know about other testers but when I use my Alphatek (on an non RCD circuit) on a no trip (soft test) setting it always gives a higher reading.

I would always note the higher reading.

Why? The high current test is much, much, much more accurate.

 

[JohnW2]

Why? The high current test is much, much, much more accurate.

That's probably true, but given two different measurement of a 'safety critical' quantity, I think a cautious person would probably feel inclined to go with the 'worse' one, wouldn't they?