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Topic Title: Maximum ZS Permitted By BS7671
Topic Summary: What is the correct value
Created On: 07 February 2015 09:00 AM
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 07 February 2015 04:26 PM
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leckie

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It might be, but it would still comply.
 07 February 2015 04:27 PM
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OMS

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Originally posted by: John Peckham

Leckie
"If you have 2.5sq.mm L and N, but only a 1.5sq.mm cpc, you could easy have a situation where the short circuit protection and volt drop are not a problem, but a higher Zs requires the RCD for fault protection".

Sounds like bad design to me!


Maybe for the Old Skool - but read 411.4.9 as Geoff points out above

We may not like it but that's harmonization for you

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 07 February 2015 04:58 PM
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phantom9

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I have been studying the Regs. alongside the Electrical Installation Design Guide and putting lots of thought in to this. I am beginning to understand this a lot more than I used to but I still have confusion in my bonce.

Zs values are directly related to the time/current characteristics of the protective device. Maximum Zs values are derived from the formula

Zs </= Uoc/Ia

Taking the example given in the EIDG for a 32 A type B this was 230/160=1.437 rounded to 1.44 in the table 41.3. AMD3 takes 95% of this because of Cmin so it becomes 1.37 ohms.

So, In order for the required disconnection time to be met, (Table 41.1) circuits not exceeding 32A must be designed such that the earth fault loop impedance (Zs) is not exceeded taking all the various design characteristics in to account.

For a TT system we cannot get Zs below the required maximum. In some cases it may not be achieved on a TN system either if the Ze is measured close to, or slightly above, the usual expected values (0.35 and 0.8) or the circuit is long. So the protective device in the event of an earth fault is unlikely to trip or disconnect quickly enough to satisfy the Regs. because of the increased impedance in the earth fault loop.

The addition of an RCD changes things for earth faults. By limiting the residual current to 30mA and confirming that the Zs is less than 1667 ohms it ensures that the touch voltage will not exceed 50V.

The confusion is this. On a test certificate we are asked to provide the maximum Zs value appropriate to the disconnection device. Under normal circumstances for TN systems we would be unlikely to encounter high impedance's but on the occasions that we do it may be possible to change the circuit characteristics by employing a lower rated disconnection device to bring the max Zs higher and bring the circuit in to limits, or we can look at limiting the length of the circuit or both. TT systems present us with high Ze/Ra which bumps up the Zs. Given that the disconnection device will handle short circuit current L/N and overload all we can do is use the RCD to limit touch voltage to less than 50V but the Max Zs to do this is 1667 ohms .

The OP presented the question in a different way and seemed to be linking the fact that it was a dual function device RCBO therefore the RCD function relates to the higher Zs.

It seems that for a healthy well designed circuit for TN systems, irrespective of the device having an RCD function, like an RCBO, the max Zs relevant is the tabulated value in 41.3. On the oher hand in a TT system it seems that the 1667 ohms in Table 41.5 is relevant. Is that the value we should record on all TT systems? I am not sure.

Edited: 07 February 2015 at 05:10 PM by phantom9
 07 February 2015 05:27 PM
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GeoffBlackwell

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Originally posted by: phantom9
It seems that for a healthy well designed circuit for TN systems, irrespective of the device having an RCD function, like an RCBO, the max Zs relevant is the tabulated value in 41.3. On the oher hand in a TT system it seems that the 1667 ohms in Table 41.5 is relevant. Is that the value we should record on all TT systems? I am not sure.


As I said ealier - there can be reasons why an RCD might be required to meet a disconnection time on a TN system - these can be due to parameters outside of the control of the contractor - such as a high Ze close but not over the limiting value for the particular type of system - it is simple to give examples

A 63A circuit on a TN-S system with a Ze of say, 0.7 ohms - even if you use a type B mcb your limit (without any corrections) is 0.69 ohms.

Also note that the 1667 ohm value from Table 41.5 is a theorectical maximum. A value that high would be unlikely to be stable. We generally aim much lower, especially if we are using rod electrodes.

Regards

Geoff Blackwell
 07 February 2015 05:35 PM
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phantom9

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With respect Geoff I am not sure you are answering my question. 63A is not relevant, either. We are talking circuits not exceeding 32A.
I know that the 1667ohms is unlikely to be present and it is derived from the formula to limit touch voltages to 50V.

50 V/0.03 A=1667 ohms

Does anyone record 1667 ohms for Max Zs on a TT system?
 07 February 2015 05:41 PM
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GeoffBlackwell

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You have stated "It seems that for a healthy well designed circuit for TN systems ....." - I have shown you an example that challenges your assertion - why is 63A not relevant, don't such circuits exist?

Regards

Geoff Blackwell
 07 February 2015 07:34 PM
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keithredpath

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Keith again. I made a mistake earlier and meant to say 1.44 Ohms. This is one ZS figure that should be entered on the condition report. Do we also enter the maximum Zs for the RCBO which is 80% of 1667 i.e. 1333 Ohms. What does Amtech enter automatically 1.44 or 1333 Ohms.

-------------------------
keithredpath
 07 February 2015 07:34 PM
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keithredpath

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Keith again. I made a mistake earlier and meant to say 1.44 Ohms. This is one ZS figure that should be entered on the condition report. Do we also enter the maximum Zs for the RCBO which is 80% of 1667 i.e. 1333 Ohms. What does Amtech enter automatically 1.44 or 1333 Ohms.

-------------------------
keithredpath
 07 February 2015 07:51 PM
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spinlondon

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Can a design which complies with the requirements of BS7671 be considered as being bad?
It may be that a better design could be produced, depending on budget constraints.
 07 February 2015 08:56 PM
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FizzleBang

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I guess EEBADS was renamed ADS to allow what was supplementary protection to now be regarded as the mode of protection.

Bang goes EEBADS cuz our European "partners" aren't quite so civiliised. I'd call that politics rather than harmonisation!

On the few occasions that I crawl out of the woodwork and fix some lectrical stuff I'll work on EEBADS and fall back on ADS as second best.
So I think that puts me in Panotom's camp...I think

-------------------------
To me, to you
 07 February 2015 09:13 PM
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spinlondon

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Where a device is used to provide both ADS and additional protection, it may be prudent to record two values.
One value for L-N faults, the other for L-E faults.
In any event whichever value is used, it should be the one stated in BS7671, not an adjusted value.
 07 February 2015 09:27 PM
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leckie

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So 1667ohms then?

I think so, even though it would be unstable, but that's what the book says.
 07 February 2015 09:30 PM
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daveparry1

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not an adjusted value

I agree with that Spin, after all the question asks for "maximum Zs allowed by BS7671".
 08 February 2015 10:00 AM
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phantom9

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Originally posted by: GeoffBlackwell

You have stated "It seems that for a healthy well designed circuit for TN systems ....." - I have shown you an example that challenges your assertion - why is 63A not relevant, don't such circuits exist?

Regards

Geoff Blackwell


Sorry Geoff. I don't want to upset you. The reason for the 32A is that the Regulation we are discussing is for circuits that do not exceed 32A.

411.3.2.2 The maximum disconnection time stated in Table 41.1 shall be applied to final circuits not exceeding 32A.
 08 February 2015 10:04 AM
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phantom9

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Originally posted by: keithredpath

Keith again. I made a mistake earlier and meant to say 1.44 Ohms. This is one ZS figure that should be entered on the condition report. Do we also enter the maximum Zs for the RCBO which is 80% of 1667 i.e. 1333 Ohms. What does Amtech enter automatically 1.44 or 1333 Ohms.


Keith I understand where you are coming from but there are two different elements to this. The Zs Max is for TN systems. The 1667 ohms is just the maximum value in Ohms Law that will ensure that the condition to limit voltage to 50V will be met. Why are you saying it should be 80% of 1667 ohms, this shows you are not understanding this basic point. Forget 80% it has nothing to do with the point you are raising.
 08 February 2015 10:09 AM
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phantom9

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Originally posted by: spinlondon

Where a device is used to provide both ADS and additional protection, it may be prudent to record two values.

One value for L-N faults, the other for L-E faults.

In any event whichever value is used, it should be the one stated in BS7671, not an adjusted value.


Regarding your first point It would be nice to see you post some reasoning, spin. You are good at nit-picking but rarely expand on a point to explain your reasoning? There is little substance to your comments, just statements.

With regards to your second point, hallelujah!

Edited: 12 February 2015 at 08:52 AM by phantom9
 08 February 2015 10:14 AM
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GeoffBlackwell

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Originally posted by: phantom9

Sorry Geoff. I don't want to upset you. The reason for the 32A is that the Regulation we are discussing is for circuits that do not exceed 32A.
411.3.2.2 The maximum disconnection time stated in Table 41.1 shall be applied to final circuits not exceeding 32A.


Your not upsetting me phantom9 - you made a general statement about RCBOs in TN systems - if you want to limit it to 32A, that is fine, my example now is a type C mcb on a TN-S system with Ze 0.7 ohms.

Table 41.3 requires that Zs should not exceed 0.68 Ohms - so how would you comply?

Regards

Geoff Blackwell
 08 February 2015 10:35 AM
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phantom9

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No I wasn't, Geoff. I am not generalising in this thread I am keeping to the OPs original question. The table 41.1 is quite specific in that it only applies to final circuits not exceeding 32A. Table 41.3 complies with 411.3.2.2 and 411.3.2.3 but for RCBOs you also need to look at 411.4.9. When you read that it takes you to table 41.5 which nicely completes the point I was trying to make.

Now to take your question in the hypothetical situation of the 32A type C breaker having max Zs of 0.68 ohms and measured Zs of 0.7 there are several different ways of adjusting the situation that are equally as hypothetical. Change it to a type B, derate the breaker to 25A, apply table 41.5 for the RCD element, rewire the circuit in larger sized cable. Who knows? The OP is asking specifically about what values to record and is not understanding the responses he is getting.
 08 February 2015 10:41 AM
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mapj1

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Hmm. We fail at the letter of the regs - perhaps we should look to the likely intention.

Now we should note it is not possible to meet voltage drop and have an L_N loop so high that an L_N fault would not clear in time. (that 63A and 0.7 ohms does not work properly if it was an L_N loop, as we would have a 50v drop on load, not likely to go unnoticed for long)

I suggest a similar line applies to a 32A circuit with a 1 ohm L_N loop
If the regs meant - "stuff the mcb related stuff, its only L_E faults we are interested in cos we think L_N faults never happen" they would say so, and the table for MCBs would not need to exist, because the L_N thing is handled by voltage drop.
But it does.
So the intention is clear - the MCB part provides ADS for an L_E fault if at all possible. So we write in the lower value.

If it fails to meet that number, and we are happy that the RCD is enough for an L_E fault, then we put in the number germane to the RCD - which is only 30mA /50V in certain situations if you being pedantic - the reduction in supply voltage does not come into that calculation - the 50V comes from an 'acceptable' touch voltage, which might equally be 25v in some places....

I agree a tick box indicating that it is intended to be MCB or RCD for LE faults would help to make things clear. As it is, if you see the low value its MCB ADS, if its 1600 ohms or whatever, then its RCB driven.

Arguably you could fit no CPC on the final circuit at all, and have a stake at the load end of the circuit and still meet the RCD ADS requirement.

Our foreign friends who don't understand why we permit reduced earth cores relative to live or neutral have a point.

-------------------------
regards Mike
 08 February 2015 10:47 AM
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GeoffBlackwell

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Originally posted by: phantom9
Now to take your question in the hypothetical situation of the 32A type C breaker having max Zs of 0.68 ohms and measured Zs of 0.7 there are several different ways of adjusting the situation that are equally as hypothetical. Change it to a type B, derate the breaker to 25A, apply table 41.5 for the RCD element, rewire the circuit in larger sized cable. Who knows? The OP is asking specifically about what values to record and is not understanding the responses he is getting.


Let us assume that the type C characteristic is required for the particular load so no option to use type B. Lets also assume that 32A is a must so no option to reduce the mcb rating.

Rewiring using a larger size cable is pointless as it is Ze that is the overriding factor.

So
It seems that for a healthy well designed circuit for TN systems, irrespective of the device having an RCD function, like an RCBO, the max Zs relevant is the tabulated value in 41.3. On the oher hand in a TT system it seems that the 1667 ohms in Table 41.5 is relevant. Is that the value we should record on all TT systems? I am not sure.


what now?

Regards

Geoff Blackwell
IET » Wiring and the regulations » Maximum ZS Permitted By BS7671

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