Steel wire armoured cables

[lyledunn]

In the UK there is extensive use of SWA cables for distribution circuits in buildings, especially at 400v using TNS, TNCS or TT earthing systems.( These are just terms given to the method employed to provide connections with earth for indirect shock protection).
It has become almost practice, albeit in most cases unnecessary, to supplement the wire armour with an additional single circuit protective conductor of the same csa as the associated phase. To some extent this was to comply with BS7671 without having to calculate the minimum csa of the protective conductor using adiabatic formula.
I was wondering if such practice is used elsewhere and what views there may be on the merits of the additional conductor. Regards,

Lyledunn

 

[Graeme38]

I think that it has some benefit for EMC, as the impedance of the SWA increases significantly with increasing frequency.

I have spent some time working overseas and I think you will find that the use of SWA cables is pretty much restricted to UK for LV distribution.

 

[bigamp]

SWA cables are commonly used in New Zealand and Australia, especially for harsh industrial or electrical hazardous area installations and for buried installations (mains cables etc.).

Common practice in previous years was to use the SWA as the earth continuity conductor (ECC) in the smaller cable sizes. I have never done the maths of it, but I believe that for smaller cable sizes the SWA is a lower impedance/higher capacity ECC than the phase conductors but for larger cables, the SWA is clearly inadequate so a separate ECC would be necessary.

It is all of accademic interest these days as wiring regulations and hazardous area installation standards demand separate ECC's (which can be part of the cable). Australian manufacturers provide 3-core plus earth, 4-core plus earth and 3-core plus neutral plus earth SWA LV (0.6/1kV) cables. 3-core LV cables tend to be available only in smaller sizes (4mmsq) intended for single phase circuits.

The SWA still ends up being used as an ECC, earthing via the cable glands, but no account is taken of the additional earthing so provided.

"SWA is OK"

Regards

 

[lyledunn]

Bigamp

Do the Regulations require a separate ecc? Certainly the UK Regulations do not.
Is the separate ecc appropriately colour coded when used as earth in a swa? Regards,

Lyledunn

 

[bigamp]

lyledunn,

Regs in NZ certainly do, not 100% sure about in Australia but suspect they do. The separate ECC is appropriately colour coded (yellow/green stripe). When we use a 4-core cable with cores coloured red, white, blue (phases) and black (neutral) for a three phase plus earth supply to a motor (no neutral required) it is OK to sleeve the black core green and use it as the ECC.

Regards

 

[Graeme38]

General rule of thumb is for cables up to about 120mm2 the cross sectional area is sufficient to be used as the earth conductor. Over that then you may have to run a separate CPC. I sometimes run a separate CPC if you are using a large MCCB (without time graded RCD protection) where you cannot get the 5 second disconnection time required for an earth fault say at the distribution board/consumer unit.

I think in the UK now there is a requirement to run an earth conductor for street lighting as part of the conductor where there is still single point earthing used.

 

[lyledunn]

Graeme38

Yes, I wonder just how beneficial the additional separate circuit protective conductor actually will be during a heavy earth fault given the electro-magnetic effects of the wire armour on the impedancce of the cpc?
I would love to see some lab test results based on such a situation. I read an article in the technical press that referred to a lab test that demonstrated almost negligible current flow in the separate copper conductor when subject to a heavy earth fault current in the swa cable. Regards,

Lyledunn

 

[Graeme38]

Lyledunn,

That is an interesting point. I have never thought about that.

 

[DanielL]

In Australia, SWA is used extensivly in mining and heavy industry. For example, in an underground mine we used SWA 11kV cable and a seperate 120mm2 earth cable. We do not factor in the use of the SWA as an earth conductor because its primary purpose is for mechanical protection. There are other practical reasons as well, if a cable uses the SWA as the only earth and the cable is disconnected for some reason, then the earth will be disconnected as well (which may effect equipment down stream leaving it unearthed).
Also, the SWA is earthed through a cable gland, which may not be as effective as a lugged cable.
For low voltage applications (up to 1kV), in most cases the cable will contain an earth core which is used for earthing purposes. The earth core is typically on the smaller side of half the size of the active conductor.

 

[dpc]

I'm not familiar with the cable construction being discussed, but if you are talking about running a separate grounding conductor that is external to the steel jacket, then Lyledunn is correct - this conductor is of little value in carrying ground fault current.

A similar issue exists with steel conduit and this was researched and tested in the U.S. back in the early 60s. The tests showed that a ground wire external to a magnetic conduit or armor was not effective in reducing circuit impedance during faults. Also, the steel conduit itself, when used as the grounding conductor has severe limitations and cannot be used for long circuits. Aluminum conduit works much better as a grounding conductor.

While the National Electrical Code still allows metal conduit to be used as a grounding conductor, most engineers require a separate grounding conductor inside the conduit. This is true for armored cable as well. I would never rely on a steel conduit or armor as a grounding conductor.

I have the references for this testing if anyone is interested.

 

[Graeme38]

Just an iteresting additional point on PVC/SWA cables - it has become practice in the UK now to replace the old 11kV paper/lead cables with PVC/SWA. The advantage of the paper/lead cable was that it acted as an auxiliary electrode and reduced the overall earth impedance at the substation. As the PVC insulates the steel armour from earth this effect is lost. If anyone upgrades old substations with this type of cable it is worth checking the earth mat resistance once again to make sure it is still within the design criteria.

 

[Ahr35181]

Quote:-

"it has become practice in the UK now to replace the old 11kV paper/lead cables with PVC/SWA"

Would anybody be able to point me to a web site which advises replacement of ageing HV PILC cables?

We recently had an 11KV PILC cable fault to earth & are considering the benefits of replacing all of them with SWA cable.

Thanks in advance......

Alan