Ferromagnetic Enclosure

Question

Hi All, 
 We came across this on a site last week and would be grateful for some clarification. 
 The panel board is supplied via single core double insulated 150mm conductors. As the picture shows, the cables enter through separate stuffing glands, but on closer inspection, the piece of metal in the centre has been slotted out so to be separate to the panel board itself. 
 As I have never seen it done this way before, could anyone confirm if this is compliant with 521.5.1 or if this could still cause an issue? 
 Regards 
 Mark

John Peckham · Accepted Answer

No issue provided the IP rating is preserved and the integrity of the DD has not been compromised. It is either that or cut a bigger aperture in the DB and fit an aluminium gland plate. 
 
 JP

mapj1 · Answer

This sort of thing can reduce magnetic hysteresis losses in the steel, by increasing the magnetic path length around any one core, encouraging the current to flow round the bundle instead. Topologically instead of multiple isolated holes you now have one, albeit an odd multi- dog-bone shape. 
 In must be said that in thin sheet steel it is rarely necessary as the magnetic heating is not that much, but as the magnetic properties of knock out box steel are not very well defined, the regs suggest you do not put flow and return currents through separate holes, and this slitting is one way to achieve that. 
 To recover the IP rating, it is possible to fill or cover the slots with another metal or indeed any material that is non-magnetic, brass braze or hard solder is one approach, but in less critical situations, filling the slots with glass-fibre, or epoxy resin or even mastic, is fine electromagnetically. Mechanically that last one may not make the grade ;-) 
 Mike

Ferromagnetic Enclosure

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