db/cu and 521.5.1 Ferromagnetic enclosures: electromagnetic effects

I don't think that there is a non-compliance.

521.5.1: Where such conductors enter a ferrous enclosure …

If the earthing conductor is bolted to a terminal which has been provided for that purpose on the outside of the enclosure, the cable does not enter the enclosure.

Chris Pearson: 
 

I don't think that there is a non-compliance.

521.5.1: Where such conductors enter a ferrous enclosure …

If the earthing conductor is bolted to a terminal which has been provided for that purpose on the outside of the enclosure, the cable does not enter the enclosure.

if this all about balancing current entering leaving…

if it [mec] goes inside to the earth bar (along with bonding etc) via a different knockout panel ?

and what is the balancing issue, if there is a problem with the above internal earthbar connection, set against bolted/glanded to the case ?

I recon that the line and neutral conductors should enter by the same hole in a ferrous enclosure to reduce the chance of heating of the cables or enclosure, but tests have proved that the heating effect of a single conductor carrying a fairly large current has a very small heating effect. Mike points out that the metal is quite thin and the heating effect is quite small if we look at it as an A.C. magnetic field cutting a ferrous conductor as in a transformer effect. Normally the earthing conductor will not carry a heavy enough current to produce any dangerous damaging heat.

Z.

Zoomup: 
 

What is the implication of all this then and with regard to the specific example(s) I used, that it is a non-compliance but ‘science’ says it doesn't really matter ?  or that it is compliant … !

I wonder if the intention behind 521.5.1 is only about thermal heating under normal conditions? The inclusion of the protective conductor in the requirement (which normally would only carry thermally insignificant currents)  might suggest they might have had something else in mind too.

I've been wondering if separating the line conductor and c.p.c. with ferrous material might increase the reactance of the circuit when large currents are involved? (Those here with a better knowledge of the underlying theory might be able to enlighten us). If so that might have the potential to reduce fault currents/increase disconnection times beyond what would be estimated from R1+R2 tests or low current Zs tests.

    - Andy.

Indeed, and it is important enough to be on the Condition Report Inspection Schedule (see 4.17). With respect to the first paragraph of the regulation, the intention seems clear but I would need to see the evidence in relation to the second paragraph. 
The third paragraph is perhaps a statement of the obvious in that it would be difficult to achieve. So are we saying that if a satisfactory response is to be given for 4.17, we need to ensure the protective conductor comes through the same aperture in a steel DB? 
The Irish regs are the same although no mention of the separate protective conductor with the swa.

521.8 of the 1981 15th edition is concerned about eddy currents. It makes no mention of the earthing conductors or C.P.C.s. It just insists on having all phase conductors and neutral (if any) contained within the same enclosure and are not separated by ferrous metal. These basic requirements extend right up to the 17th edition, with no mention of earthing conductors or C.P.C.s.

So, what has changed between then and now? S.P.D.s?

Z.

My copy of the 16th Ed (red cover) 1991 mentions ‘and the appropriate protective conductor’ in 521-02-01 - so well before SPDs.

   - Andy.

AJJewsbury: 
 

My copy of the 16th Ed (red cover) 1991 mentions ‘and the appropriate protective conductor’ in 521-02-01 - so well before SPDs.

   - Andy.

Ooooops, yes so it does. My mistake.

Z.