Why does SWA cable have to have to be earthed?

[^woody^]

What is the reasoning behind the requirement for the strands of 3 core SWA cable to be earthed, when there is already an earth cable within it?

It seems that the cable within the armouring will carry an earth to the fitting/appliance, so why does the cable need this additional earth connection?

 

[stuartsjg]

The Armour can be used as the appliance protective earth, such that for single phase you only need 2 core for live and neutral.

For 3 phase you use either 3 or 4 core (depending on balanced/unbalanced load) . You rarely see a copper core for an earth connection in a 3 (3 or 4 wire) phase system as the armour is nearly always the earth.

Often single phase connections will use 3 core as its nearly the same price as 2 core and the electrician will use the 3rd core as earth.

You connect the armour to earth so that should the cable be penetrated, the chances are the fault current will go down the armour and trip the protection.

Think of a nail going through the cable. It will contact the armour then a core. If the armour wasn't earthed then the nail would become live.

Stuart

 

[ColJack]

so that if you stick a nail / screw / garden fork through it and hit a live wire, it shorts out and blows the fuse / breaker.

otherwise your nail / screw / garden fork becomes live

 

[Spark123]

Also because using a core the same size as a live conductor will comply with table 54.7.
Using the armour as the only CPC is a little bit more complex and involves a little more brain power to see if it complies with the regs hence if someone is feeling lazy it is easier to double up and use the 3rd core as the CPC.

 

[ColJack]

he's not asking that, he's asking why earth the SWA if it's already got a core dedicated as asn earth..

 

[Spark123]

The answer is you can use 2 core SWA and just use the armour as the earth - it is only because people like to be lazy and buy 3 core to avoiding doing the calcs that they need a spare core for the earth inside the cable.

If the armour is being used as the CPC then there is often no need to use an inner conductor as a CPC - but we earth them anyway to ensure they are not floating. This also has the added benefit of it automatically complying with 54.7.

I can't ever remember seeing SWA with a green/yellow core as such - the normal 3 core cable I am thinking of is manufactured for 3 phase use with the cores sleeved at termination.

 

[ColJack]

again, that's not what he's asking and could care less about adiabatic equations on the SWA itself...

he's asking why earth the armour if it's already got an earth core in it...

the answer to which is as above... job done, question answered, stop trying to make it more complicated just to make yourself sound superior in your knowledge..

 

[^woody^]

But if the cable is protected by an RCB, then wouldn't it trip anyway if broken?

I was just thinking that where a property has RCB protection, all the other cables around a house are protected if broken, so why does SWA need this "extra" protection.

Is it purely due to extra risk due to being outside - although those extra risks are presumably dealt with by installing the cable deep(er) underground or high enough out of harms way?

 

[toasty]

again, that's not what he's asking and could care less about adiabatic equations on the SWA itself...

he's asking why earth the armour if it's already got an earth core in it...

the answer to which is as above... job done, question answered, stop trying to make it more complicated just to make yourself sound superior in your knowledge..

Steel Wire Armoured Cable, commonly abbreviated as SWA, is a hard-wearing power cable designed for the supply of mains electricity. It is one of a number of armoured electrical cables - which include 11KV Cable and 33KV Cable - and is found in underground systems, power networks and cable ducting.

The typical construction of a SWA Cable can be broken down as follows:
Conductor: consists of plain stranded copper (cables are classified to indicate the degree of flexibility. Class 2 refers to rigid stranded copper conductors as stipulated by British Standard BS EN 60228:2005)

Insulation: Cross-linked polyethylene (XLPE) is used in a number of power cables because it has good water resistance and excellent electrical properties. Insulation in cables ensures that conductors and other metal substances do not come into contact with each other.
Bedding: Polyvinyl chloride (PVC) bedding is used to provide a protective boundary between inner and outer layers of the cable.
Armour: Steel wire armour provides mechanical protection, which means the cable can withstand higher stresses, be buried directly and used in external or underground projects. While the steel armour can be used as an additional earth for the cable, it should not be used as the only earth. The armour must be earthed at the supply end at all times,however, so that it cannot become live if the cable is damaged.
Sheath: a black PVC sheath holds all components of the cable together and provides additional protection from external stresses.
The PVC version of SWA Cable,described above, meets the requirements of both British Standard BS5467 and International Electrotechnical Commission standard IEC 60502. It is known as SWA BS5467 Cable and it has a voltage rating of 600/1000V.
SWA Cable can be referred to more generally as Mains Cable, Armoured Cable, Power Cable and Booklet Armoured Cable. The name Power Cable, however, applies to a wide range of cables including 6381Y, NYCY, NYY-J and 6491X Cable.

(Source Wikipedia)

You'll be telling me that lot above isn't relevant next...

 

[bernardgreen]

But if the cable is protected by an RCB, then wouldn't it trip anyway if broken?

It would trip ONLY if the break resulted in a Live to Earth leakage greater than the trip level of the RCD. If the wire armour was not earthed then and the Live made contact with the amouring the armour would be Live and the RCD would still not trip.

So the armour is now lethally dangerous. Someone touching it and also in contact with the ground would receive a severe shock. Possibly fatal.

 

[Spark123]

again, that's not what he's asking and could care less about adiabatic equations on the SWA itself...

he's asking why earth the armour if it's already got an earth core in it...

Fact is that it is the opposite way around, the [extra] core which can be used for the earth which is extra - not the SWA. The SWA should be earthed regardless.

the answer to which is as above... job done, question answered, stop trying to make it more complicated just to make yourself sound superior in your knowledge..

Nope - I am telling it as it is. The armour is the CPC in SWA, anything else is supplementary. I thought you would have known that?

 

[ericmark]

Where the cables is below 6mm three core the cross sectional equivalent area does not exceed the 10mm required for main protective bonding conductors. Also as the cables exceed 95mm you will also find the armouring is not big enough. John Peckham from IET did a great article on use of wire armour as an earth.
I remember we always stocked 3 core SWA 6mm and it was cheaper than many of the other sizes near to it because so many people did the same and if 6mm would do we used that and only ordered cables when for some reason 6mm three core would not fit the bill.
As already said it does not matter if a core is used as earth or not the armour is always earthed at the supply. Some companies insist on a "Pressure test" where the earth is disconnected from armour for test and the resistance between armour to earth is measured. This should be high. But if the cable has been damaged by digging in the area it will become low. Company who did digging is then charged for replacement. Big problem is where the cable outer sheath has become damaged it can rust and the earth can be lost.
Earthing armour at source only can reduce EMC problems and sometimes this is done but normally only where there is a lot of RF floating around. Transmitting sites etc.

 

[securespark]

the answer to which is as above... job done, question answered, stop trying to make it more complicated just to make yourself sound superior in your knowledge..

What's this all about?

I've read a few threads recently where you have clashed with Spark 123 about what he has written. that in itself is not a problem (although I have not been able to understand your gripe), but you then start chucking insults...

Chill out man...live & let live.

 

[sm1thson]

I always find it helpful when the pros add some additional information. I find this because 'a little knowledge is a dangerous thing’ so its good to know some of the other things surrounding the original question to prevent further assumptions been made. Also what the OP ask's and what they mean are often 2 different things, as is what they are then going to go do with this new found knowledge.

 

[ricicle]

the answer to which is as above... job done, question answered, stop trying to make it more complicated just to make yourself sound superior in your knowledge..

What's this all about?

I've read a few threads recently where you have clashed with Spark 123 about what he has written. that in itself is not a problem (although I have not been able to understand your gripe), but you then start chucking insults...

Chill out man...live & let live.

Remember Col is not sleeping, grouchy perhaps ?