recorded cpc size

MDJ · Mar 29, 2014

I have also been in factories where the trunking is so old and also hanging off the wall to consider using it as a CPC would have been ridiculous. That said a new metal containment system I agree should not need seperate CPCs.

Engineer54 · Mar 29, 2014

telectrix said:
so, just to play devil, isf as you say, eng., that the containment acting as cpc is superior to a copper cable, then by that you mean that the containment has a lower R2 value. therefore the containment will carry a larger proportion of the fault current than the copper cable.

Correct, if the metal containment meets the CSA requirement in it's own right and not as a combined figure with a copper conductor, then the metal containment will be the main CPC and therefore carry the larger proportion (if not virtually all) of the fault current

We are talking separate CPC's, so you will only be measuring ONE CPC per circuit, average is what between 1 mm to 2.5 mm for most final circuits, whereas even the smallest containment is many times that figure and even allowing for the 8X conductivity difference, will still outperform that of a single separate CPC...

Jesus, i'm getting really tired now, so i hope i'm still making sense, and not drifting off course...lol!!

telectrix · Mar 29, 2014

pepparz said:
In the past I have had to rewire factory lighting because the continity of conduit has failed and there's been no cpc so had to rewire in swa 2 week job on a cherry picker.if there had been a internal cpc this wouldn't have happened.

your other options would have been:

1. fish a cpc through the conduit.

2. repair the conduit as cpc.

Engineer54 · Mar 29, 2014

pepparz said:
I agree to a point but go back in 25 years and check zs!

MDJ said:
I have also been in factories where the trunking is so old and also hanging off the wall to consider using it as a CPC would have been ridiculous. That said a new metal containment system I agree should not need seperate CPCs.

Some of the companies factories where i did my training were nigh on 40 years old, as was the metal electrical containment systems. Hand on heart, in all the time i was with that company i never came across any containment trunking or conduit etc, that failed either Zs or high current continuity testing (old Clare conduit tester)!! And believe me, all building installations were tested on a regular ongoing rota, as were factory machinery and equipment...

Never been a more true statement than that Gaz made above....

"If you need to put a CPC in steel conduit, then you can't do steel conduit properly!"

MDJ · Mar 29, 2014

Engineer54 said:
Some of the companies factories where i did my training were nigh on 40 years old, as was the metal electrical containment systems. Hand on heart, in all the time i was with that company i never came across any containment trunking or conduit etc, that failed either Zs or high current continuity testing (old Clare conduit tester)!! And believe me, all building installations were tested on a regular ongoing rota, as were factory machinery and equipment...

Never been a more true statement than that Gaz made above....

"If you need to put a CPC in steel conduit, then you can't do steel conduit properly!"

Yes I agree of course, we are told to install cpcs in steel conduit by engineers who work for the council mind, and although there is no need we do as we are told unfortunately.

Engineer54 · Mar 29, 2014

MDJ said:
Yes I agree of course, we are told to install cpcs in steel conduit by engineers who work for the council mind, and although there is no need we do as we are told unfortunately.

Either Kids or seasoned Engineers that don't trust modern day electricians to adequately construct metal containment systems.... But it's far more likely the case, to find what i call ''Failed Engineers'' working for local councils. Those that just can't hack it professionally in the real world!! lol!!

Badged01 · Mar 29, 2014

Engineer54 said:
Think about your electrical training, and you'll see that it doesn't and can't work like that!!

Not seeing where you're coming from here. If I have two conductors joined at either end then the current will be split in some proportion between the two of them as they are parallel? Many moons ago (probably early 2000?) I also remember reading a technical report that suggested exactly this for running a parallel CPC conductor outside the armour of SWA. (Cant think of the report but Im sure with digging across the internet it could be found somewhere in the archives!)

Of course this may all be hypothetical as until you get above 100mm'ish(?) cores the SWA the armour is sufficient in its own right to be used as a CPC.

pepparz · Mar 29, 2014

Thanks for ur comments eng but I got clarification on post #2

pepparz · Mar 29, 2014

Badged01 said:
Not seeing where you're coming from here. If I have two conductors joined at either end then the current will be split in some proportion between the two of them as they are parallel? Many moons ago (probably early 2000?) I also remember reading a technical report that suggested exactly this for running a parallel CPC conductor outside the armour of SWA. (Cant think of the report but Im sure with digging across the internet it could be found somewhere in the archives!)

Of course this may all be hypothetical as until you get above 100mm'ish(?) cores the SWA the armour is sufficient in its own right to be used as a CPC.

Unless its for bonding purposes

Badged01 · Mar 29, 2014

pepparz said:
Unless its for bonding purposes

Lost me there as well!

pepparz · Mar 29, 2014

Swa armourings

Engineer54 · Mar 29, 2014

Badged01 said:
Not seeing where you're coming from here. If I have two conductors joined at either end then the current will be split in some proportion between the two of them as they are parallel? Many moons ago (probably early 2000?) I also remember reading a technical report that suggested exactly this for running a parallel CPC conductor outside the armour of SWA. (Cant think of the report but Im sure with digging across the internet it could be found somewhere in the archives!)

Of course this may all be hypothetical as until you get above 100mm'ish(?) cores the SWA the armour is sufficient in its own right to be used as a CPC.

For that situation to work, Both the copper and the steel need to be of an adequate size to carry the full fault current, end of!! Using one to back up the other to achieve a meaningless CSA measurement is not an option and never has been... The separate parallel copper CPC needed to be sized to carry the ''FULL'' fault current of the circuit it is protecting ''NOT'' just a portion of it...

Virtually all SWA configurations (eg ...number of cores) will comply using the SWA as the CPC up to 95mm The first cables to drop away from compliance is the 2 core cables.

Bonding requirement CSA's will need to comply with copper equivalents, so in most cases, the SW armouring will not comply with the relevant regulations....

Engineer54 · Mar 29, 2014

Note: These tables relate to compliance for CPC only, ...NOT for Main Bonding purposes...
[h=3]SWA/CPC tables[/h]

Blue = min CSA of cables SWA to comply.

(Red) = Cable does not comply.

Table for 70[SUP]0[/SUP]C Thermoplastic PVC SWA cables.

Conductor CSA	Minimum CSA of SWA to meet 54G	CSA of armour 2 core	CSA of armour 3 core	CSA of armour 4 core
1.5	3.4	15	16	17
2.5	5.7	17	19	20
4	9.0	21	23	35
6	13.6	24	36	40
10	22.6	41	44	49
16	36.1	46	50	72
25	36.1	60	66	76
35	36.1	66	74	84
50	56.4	74	84	122
70	79.0	84	119	138
95	107.2	122	138	160
120	135.3	(131)	150	220
150	169.2	(144)	211	240
185	208.6	(201)	230	265
240	270.6	(225)	(260)	299
300	338.3	(250)	(289)	(333)
400	403.9	(279)	(319)	467

Table for 90[SUP]0[/SUP]C Thermosetting SWA cables operating at 70[SUP]0[/SUP]C.

Conductor CSA	Minimum CSA of SWA to meet 54G	CSA of armour 2 core	CSA of armour 3 core	CSA of armour 4 core
1.5	3.4	16	17	18
2.5	5.7	17	19	20
4	9.0	19	21	23
6	13.6	22	23	36
10	22.6	26	39	43
16	36.1	41	44	49
25	36.1	42	62	70
35	36.1	62	70	80
50	56.4	68	78	90
70	79.0	80	90	131
95	107.2	113	128	147
120	135.3	(125)	141	206
150	169.2	(138)	201	230
185	208.6	(191)	220	255
240	270.6	(215)	(250)	289
300	338.3	(235)	(269)	(319)
400	451.0	(265)	(304)	452

Badged01 · Mar 29, 2014

Engineer54 said:
For that situation to work, Both the copper and the steel need to be of an adequate size to carry the full fault current, end of!!

I'll agree to disagree then.

There's an IET document (BS7671) that permits its use and a technical report commissioned for the ECA that futher substantiates the fact that an external CPC can be run parallel to the armour.

Badged01 · Mar 29, 2014

Well being a boring old fa*t on a Saturday afternoon I dug a bit deeper into all this CPC stuff and have even stumped myself.

BS7671 allows a seperate CPC to be run in addition to the armour of SWA and having found my report from the ECA (2007) it recommends that the external CPC should be a minimum of a 1/4 of the size of the line conductor. So far so good for me assuming it hasn't been updated? (Clearly as a seperate CPC it has to be a min of 4mm if not protected)

Then I took a cursory glance thro the 'green' GN8 to see if any light is shed on it in there. Lo and behold GN8 'recommends' that the external CPC is sized to take the full fault current.

So to practise what I preach in that you always cover your b*tt by using BS7671 for work you do (and by implication GN8), I am now going to do a 180 deg turnaround and agree that if you are going to run an external CPC you size it for the expected fault current.

Dog, old, trick, new has happened ......