gkenyon: 
 

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Hello G A, We are using the 18th edition nowadays. Many times the armour is adequate to use as a C.P.C.

Use SWA as CPC, a guide to the acceptability of steel wire armour use as CPC. (gadsolutions.biz).

Z.

Just to note that the calculation method for loop impedance shown in Table 4 on this site, whilst still demonstrated in the latest (2018) IET Electrical Installation Design Guide, should really be replaced for cables with line conductor csa 16 sq mm and above (where reactance comes into play more) by the method shown in Annex NA of PD IEC/TR 50480:2011 - specifically sections NA 4.4 (armour alone used as cpc) and NA 4.5 (external cpc). 

For the case where the armour is used as cpc, the total loop impedance contribution of the SWA cable for a line to earth fault is given by:
 

R1 is the DC resistance milliohms per metre) of the line conductor, R2 is the DC resistance (milliohms per metre) of the SWA.
 

Don't forget the above is in milliohms per metre, so multiply by length, divide by 1000. 

This leads to a slightly higher overall DC resistance per metre but a slightly lower overall reactance per metre.

 

 

Thanks. How will my suggested upfront S type 300mA R.C.D. affect the overall situation?

Z.

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Thanks. How will my suggested upfront S type 300mA R.C.D. affect the overall situation?

 

Z.

RCDs do not provide protection against overcurrent … In a TN system it's a little easier to address protection against overcurrent based on adiabatic, or manufacturer's let-through energy. In a TT system, to fully answer the question, you would need to know the maximum prospective earth fault current (that occurs when all extraneous-conductive-parts are connected in TT, as opposed to max Ze which is the fault through the earth electrode system alone in a TT system) … 

gkenyon: 
 

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Thanks. How will my suggested upfront S type 300mA R.C.D. affect the overall situation?

 

Z.

RCDs do not provide protection against overcurrent … In a TN system it's a little easier to address protection against overcurrent based on adiabatic, or manufacturer's let-through energy. In a TT system, to fully answer the question, you would need to know the maximum prospective earth fault current (that occurs when all extraneous-conductive-parts are connected in TT, as opposed to max Ze which is the fault through the earth electrode system alone in a TT system) … 

I am considering the armour as being suitable as a C.P.C. In this context surely an earth fault will clear very quickly if an R.C.D. is installed at the origin of the S.W.A. cable, and disconnect the fault before serious damage due to cable  heating can occur.

If the cable is supplying a stable then the fault current may only be a few hundred Amps as buried metal will offer a poor earth return.

Z.

In this context surely an earth fault will clear very quickly if an R.C.D. is installed at the origin of the S.W.A. cable, and disconnect the fault before serious damage due to cable  heating can occur.

Fuses and MCBs can be significantly faster than RCDs at high fault currents - an S type won't open in less than 40ms - at say 6kA that's 1,440,000 A2s so by the adiabatic would need over a 10mm2 copper conductor (if k=115).

By comparison a B-type MCB at 6kA should have an energy let-though of less than 54,000 A2s - so should be adequate with just a 2.5mm2 conductor from a fault protection perspective.

   - Andy.

AJJewsbury: 
 

In this context surely an earth fault will clear very quickly if an R.C.D. is installed at the origin of the S.W.A. cable, and disconnect the fault before serious damage due to cable  heating can occur.

Fuses and MCBs can be significantly faster than RCDs at high fault currents - an S type won't open in less than 40ms - at say 6kA that's 1,440,000 A2s so by the adiabatic would need over a 10mm2 copper conductor (if k=115).

By comparison a B-type MCB at 6kA should have an energy let-though of less than 54,000 A2s - so should be adequate with just a 2.5mm2 conductor from a fault protection perspective.

   - Andy.

But I am not considering high fault currents. The TT earthed stable will probably not allow a high L to E fault current. So the R.C.D. will be king. Fuses won't blow and M.C.B.s won't trip.

Z.

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AJJewsbury: 
 

In this context surely an earth fault will clear very quickly if an R.C.D. is installed at the origin of the S.W.A. cable, and disconnect the fault before serious damage due to cable  heating can occur.

Fuses and MCBs can be significantly faster than RCDs at high fault currents - an S type won't open in less than 40ms - at say 6kA that's 1,440,000 A2s so by the adiabatic would need over a 10mm2 copper conductor (if k=115).

By comparison a B-type MCB at 6kA should have an energy let-though of less than 54,000 A2s - so should be adequate with just a 2.5mm2 conductor from a fault protection perspective.

   - Andy.

But I am not considering high fault currents. The TT earthed stable will probably not allow a high L to E fault current. So the R.C.D. will be king. Fuses won't blow and M.C.B.s won't trip.

Z.

As I said in my reply, you will have to determine the maximum earth fault current, and determine the necessary overcurrent protection of the cpc in line with that.

First, don't use the measured Ze to the earth electrode alone to determine the maximum prospective fault current. When extraneous-conductive-parts are connected (if any), the effective combined earth electrode resistance may well be far lower than you measure doing the Ze test.

Example - suppose the Earth electrode resistance is 70 ohms, and effective resistance of extraneous-conductive-parts 10 Ohms. Combined resistance =70*10/(70+10) = 8.75 ohms. 
You thought you only had a fault current of 3.3 A, but in fact you have a fault current of 26.2 A.
Not a problem if your cable has a current-carrying capacity of at least 26 A (4 sq mm copper). But if the RCD didn't trip, would you be OK with 1.5 or 2.5 sq mm cpc, say for a fault near the origin?
I think in this case, we'd all agree the RCD would operate before the cpc had an issue.

However, what if the maximum earth fault current is much greater … for example:

(a) the effective combined earth electrode resistance of the extraneous-conductive-parts is 1 ohm? We've now got (at the origin) over 200 A of prospective fault current. We can't rely on that for protection against electric shock (we can only rely on the loop impedance through the means of earthing - the consumer earth electrode - for that) ,,, but we need to take it into account for overcurrent protection, and an RCD cannot provide overcurrent protection even for earth faults.

(b) the TT system is derived from a TN system, and unfortunately shares extraneous-conductive-parts with that system. Again, can't rely on that for protection against electric shock … but this time we definitely need to size our cpc's in a similar manner to TN systems for protection against overcurrent (earth fault current). Again, the RCD is not an overcurrent protective device according to BS 7671, it requires back-up of a suitable overcurrent protective device, and the cpc sized accordingly.


Not so clear-cut, is it? 

QUOTE: “(b) the TT system is derived from a TN system, and unfortunately shares extraneous-conductive-parts with that system. Again, can't rely on that for protection against electric shock … but this time we definitely need to size our cpc's in a similar manner to TN systems for protection against overcurrent (earth fault current). Again, the RCD is not an overcurrent protective device according to BS 7671, it requires back-up of a suitable overcurrent protective device, and the cpc sized accordingly.”

But if the stable has no extraneous-conductive-parts and is of wooden construction, perhaps with a blue plastic water pipe only, that puts a different complexion on things.

Z.
 

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QUOTE: “(b) the TT system is derived from a TN system, and unfortunately shares extraneous-conductive-parts with that system. Again, can't rely on that for protection against electric shock … but this time we definitely need to size our cpc's in a similar manner to TN systems for protection against overcurrent (earth fault current). Again, the RCD is not an overcurrent protective device according to BS 7671, it requires back-up of a suitable overcurrent protective device, and the cpc sized accordingly.”

But if the stable has no extraneous-conductive-parts and is of wooden construction, perhaps with a blue plastic water pipe only, that puts a different complexion on things.

Z.
 

It's very difficult … you know the site layout and the earthing arrangements, sadly we don't.

Does the stable have its own earth electrode and effectively you're only exporting live conductors? Or are you exporting the protective conductor from another part of the installation that does have extraneous-conductive-parts?

And regardless …  even if it has its own electrode and only imports live conductors, you know the earth electrode resistance (whether it's 1 ohm with Ipef = 230 A, or 100 ohms and Ipef = 23 A) and we don't.

And of course, I don't know whether the stable has a concrete floor or wooden floor … if concrete, re-bar recommended to be main-bonded regardless of supply earthing arrangement (not just PME) see 705.415.2.1.

gkenyon: 
 

Zoomup: 
 

QUOTE: “(b) the TT system is derived from a TN system, and unfortunately shares extraneous-conductive-parts with that system. Again, can't rely on that for protection against electric shock … but this time we definitely need to size our cpc's in a similar manner to TN systems for protection against overcurrent (earth fault current). Again, the RCD is not an overcurrent protective device according to BS 7671, it requires back-up of a suitable overcurrent protective device, and the cpc sized accordingly.”

But if the stable has no extraneous-conductive-parts and is of wooden construction, perhaps with a blue plastic water pipe only, that puts a different complexion on things.

Z.
 

It's very difficult … you know the site layout and the earthing arrangements, sadly we don't.

Does the stable have its own earth electrode and effectively you're only exporting live conductors? Or are you exporting the protective conductor from another part of the installation that does have extraneous-conductive-parts?

And regardless …  even if it has its own electrode and only imports live conductors, you know the earth electrode resistance (whether it's 1 ohm with Ipef = 230 A, or 100 ohms and Ipef = 23 A) and we don't.

And of course, I don't know whether the stable has a concrete floor or wooden floor … if concrete, re-bar recommended to be main-bonded regardless of supply earthing arrangement (not just PME) see 705.415.2.1.

Hello Graham, I am just making assumptions based upon the O.P. and personal experience. Your deeper insights have though been beneficial to me. Thanks.

Z.

gkenyon: 
And of course, I don't know whether the stable has a concrete floor or wooden floor … if concrete, re-bar recommended to be main-bonded regardless of supply earthing arrangement (not just PME) see 705.415.2.1.

Does Section 705 always apply to stables? The picture that I got from the OP was that the stables and garage were in a domestic setting.