isolators for earth bar or not?

Question

Can someone explain why Erath bar is sitting on an isolators and not directly on the backplate creating solid earthing connection to the whole box/panel/enclosure? 
 I remember in my old place we bolted in PE bars directly on a backplate but in my present company we use isolators. 
 Is this normal procedure or only necessary in some circumstances?

Cheers

gkenyon · Accepted Answer

There are TWO very good reasons why earth bars are installed on insulated mountings: 
 1. To better-control corrosion. Can be problematic for DC 'protective conductor currents' ('leakage currents') and also with certain frequencies. 
 2. For EMC purposes: the 'star point' earthing approach of BS IEC 61000-5-2 helps stop 'earth loops' which become important for higher frequency "noise". If you want the enclosure and/or backplate to be an EMC shield, it's better to individually connect each of the "panels" (front, back, left side, right side, top, bottom), each shelf/backplate, etc., through a copper braid or round conductor of between 6 and 16 sq mm. Similarly, with current-using equipment, if you connect cpc's back to the star point (vie the mains distribution units) in a separate way, rather than having "leakage currents" flowing around the cubicle itself, and also through any functional earthing, works best. Also, if CPFE (combined protective and functional earthing) is employed, the star point arrangement helps prevent PE currents flowing through purely FE connections, using the earth bar as the "earth reference" for that. Correctly-applied star-point earthing in an overall common-bonding network can be particularly important for equipment in installations with inverter drives. 
 It would be important to take this into account, especially in the UK and EU countries, for conformity to the EMC DIrective (EMC Regulations 2016 in the UK).

isolators for earth bar or not?

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