Cables in parallel

[Darkwood]

You are 100% correct (blonde moment)... but really never needed to do it anyway as it dosn't become an issue until you use the big cables and i rarely exceed 70mm 4cores in swa but thinking back many years we did do a phase csa matching earth on a 185mm, scared myself there how things just leak away from your grey matter if not part of your day to day - yr to yr work.
Ill stand up though and take this one on the chin

 

[Engineer54]

Darkwood, ... Don't we ALL have blonde moments, the older we get the more we have!! ...lol!!

By the way, 4 core 185mm SWA cables will comply with CPC requirements, but 2 and 3 core won't...:yesnod: lol!!

 

[Darkwood]

Well just glad that 99% of the crap i dish out is good advice but i was due that 1% lol....

 

[Silly Sausage]

Errr, you don't run a separate earth to make up the difference of a non-compliant SWA cable, you install a separate CPC of the correct size to meet compliance of the line conductors!! You don't mix steel and copper as a CPC at any time...

There's more chance of me winning the lottery than that of installing a cable of this size, but I've now got a question rattling around inside my skull.
If a separate CPC is required, would the armour of the SWA need to be isolated from earth at one its ends?

 

[johnboy6083]

i read a report by the ECA a while ago that stated that for CPC/ earthing conductors, the current is shared between parallel conductors, with a ratio closely matching the ratio between the size of cable used. So in other words, this report ( if i remember correctly) says what darkwood stated earlier is correct. I will try to find this, before i or anyone else takes it as gospel though

 

[Guest123]

I will try to find this, before i or anyone else takes it as gospel though

I have it on file.

 

[johnboy6083]

I have it on file.

could you post mate, im at a dead end tying to find it.

 

[Darkwood]

Be great if someone could fill the hole i dug myself lol.... reason i put it was how we were taught at college and if i recall an ECA report did state that a fault could overload additional earthing if it wasn't large enough but they then followed with guidence the additional earthing must not be less than a quarter of the phase csa.
Ive seen the additional earthing senerio to make the difference up used on many occasions on various sites as a means to reduce costs and seems perfectly acceptable to clark of works to implement this method... as mentioned before the sizes i use dont usually fall into this area but be good to know what the regs state on this issue.

 

[Guest123]

There are very much two schools of though on this.

The report commissioned by the ECA and carried out by the ERA agrees with darkwood in so much as an additional CPC can be run to supplement the armour of an SWA where the earth fault loop impedance falls short of the mark.

Other highly respected industry members believe that if an additional CPC is needed, it should be large enough to cope with the maximum fault current expected, by itself as you cannot guarantee what percentage of the fault current will flow in the armour or the additonal CPC.

I have the reports from both camps and will add them when I get on my PC later.

Personally, I'm in the 'if you're going to run an additonal CPC, run one big enough to carry out the fuction by itself' camp.

 

[Darkwood]

As i say Len it just my teachings through the 15th edition but im thus divided it can be assumed the fault current is self regulating and research has shown it to be so in that any current flowing through a cable will create a temp rise and susequently resistance and where a parallel path exist it will share the current accordingly so as to maintain lowest overall resistive path but as twin socket has shown with the link there is an issue of generated magnetic fields, the issue is argued constantly as you say from different bodies but i have not ever heard or recollect any mention of a additional earthing to make the csa up failing due the current deciding to use it solely as return path because that dosn't happen. What i believe is been done here is possible issues with glanding, corrosion etc of the swa joint (which i guess you have seen your fair share of) will give presidence to the additional earth as the major path so a play of caution in the industry is put on this subject.

Another point is we were told if used with a moulded case circuit breaker then it was acceptable as tripping time would almost be instantaneous

The industry through the 15th and partly 16th was one that didn't test installs as part of the job, now times are changing and the new crop of sparkies are systematically testing all they do this argument may once again swing back to earthing to make up the difference, especially in time of high copper costs and global recessions.

 

[johnboy6083]

Ive just read the report , try this: http://www.google.co.uk/url?sa=t&rct=j&q=eca%2F%20era%20report%20on%20conductors&source=web&cd=1&ved=0CEsQFjAA&url=http%3A%2F%2Fwww.wiley.com%2Flegacy%2Fwileychi%2Feca_wiringregulations%2Fsupp%2FAppendix_16.pdf&ei=cqCuT9KFG4LU0QWzhai0CQ&usg=AFQjCNEm0O1r9ZSztXLNacYz8Fyv7PFnEQ

to summarize, the only cables that will not meet disconnection times using armour as CPC are: 120 and 400mm2 2 cores. These are rare cables to install anyway.
If by bad design, you find that your Zs values will not be met, then a CPC at least 1/4 of the pahse conductor can be used.

IMO, if your using very large cables, then they are either to feed DB;s or large machines.

If your feeding a DB in an envbiroment with little or no extraneous metal, then i would probably use an external full size CPC, in order to keep the size of my final circuits down in order to meet Zs values. In an environment where there is lots of extraneous metalwork, then i would probably be happy to use the armour as CPXC, providing the envromental conditions allowed.
If your feeding a large machine opr motor, then fruther manufacturers guidance would be used, and the use of an external CPC would probably be neccesary.
All of the large pumps i install, have external CPC's that are smaller than the phase conductors. But they also have lots of other SWA cables running in paralell. The distibution of fault currents cant be equal, or all of the cables would fry. On one job i fed a 160Kw motor with 150mm scpre, and it had a 25mm2 external CPC. the fuse size was 315A, and it was fed via a VSD.

 

[Silly Sausage]

Personally, I'm in the 'if you're going to run an additional CPC, run one big enough to carry out the function by itself' camp.

I suspect that's what a certain Chinese resident person will probably agree with.

 

[Darkwood]

Its clear with different bodies out there having differing views there wont be any clear resolve to this debate and in that case and until proven without doubt (on the acceptance of earthing to make up the difference of a armour that falls short) ill obviously have to ignore my teachings and go with the method that is known to be adequate; so matching the csa of the phase conductor is the method that takes the crown.

As i said though, its an swa size i dont use and its just for knowledge basis only, it makes the grey matter twitch and adds to my ever increasing understanding of my job even if this is still a debatable grey area.

 

[Engineer54]

I'll have to read this ERA report again. I speed read it last night, and it seems they were changing bits and Bob's to suit their measured values on a 7 metre test rig!! Not sure what they were up too, but a 7 metre test rig isn't going to relate too much, to a circuit measuring in the 10's of metres. According to this report only 2 SWA 2 core cables were found not suitable, where i would have said that all 2 core SWA cables over 95mm wouldn't comply. As i said i'll have to go through this report again...lol!!

I wouldn't agree at all, that the use of 120mm SWA cables are rare. 120mm SWA cables are a very common size on commercial and industrial installations, and commonly used for parallel runs replacing 300mm single cable. Probably less common mind, in 2 core configuration... lol!!

 

[Guest123]

I'll have to read this ERA report again.QUOTE]


I can e-mail you the reports from both camps if you'd like.

 

[Engineer54]

I'll have to read this ERA report again.QUOTE]


I can e-mail you the reports from both camps if you'd like.

That would be very useful Lenny thanks!! Always better to see both sides of the coin so to speak!! lol!!!

I take it you have the/my e-mail address i use with the forum??

 

[Guest123]

I take it you have the/my e-mail address i use with the forum??

I'll PM you.