Cables in parallel

[Engineer54]

A cable run in parallel is classified as a Single cable!! The rules are that both cables must be of the same CSA and of the same type/construction, be the same length, (run in the same containment or follow the same route). Though what your suggesting is compliant, it is somewhat on the over kill side of things...lol!!!

 

[Darkwood]

Approach this from another angle you have a swa cable and your armour isnt large enough for the earthing required, solution is to run a seperate earth with it to make up the additional earthing needed, in this case you dont have to match size to size and all your required to achieve is the csa of earthing needed is met. The earth is usually only carrying a large current in a short circuit and this will be for a very short period (hopefully) so assume it need not follow the same rules placed on load bearing cables in parallel in that they must match size for size to equally distribute the load, or you may end up running a oversized earth with a 120mm swa when you may only need a 25mm to make up the difference.

 

[Engineer54]

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...

 

[Des 56]

The seperate earth conductor would need to withstand the full fault current


If you use a separate g/y with an swa then it must be capable of carrying that full fault current,it cant be added for size purposes

 

[KINKY]

Have a read of this, interesting.

 

[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.