Mobile Display Units Earthing Requirements

[electricalvillage]

Is anyone up on earthing requirements for mobile display units supplied by an onboard generator. I've looked at 711 and 717 and I'm left confused by the earth connection to the chasis of each vehicle. There are RCDs on these vehicles. Confusion comes from key to figures 717.1 to 4 '7e to an earth electrode for protective purposes if required'

All help gratefully received.

 

[sparkytim]

Often difficult for them to get an earth Rod in so they sit as a stand a lone installation fed of the on board genny with no reference to Earth.

 

[electricalvillage]

Thanks for that sparkytim. Do you know if the RCD's work in these circumstances?

 

[Andy™]

depends where your supply is from....

if its on-board, then RCD will trip between live conductor and vehicle (if its earthed). if resistance to ground is low enough, then it could work. but if resistance is too high for RCD, then it may also be too high for a shock (IT supply)

if its a DNO supply, then ground will be adequately earthed and RCD should work. but if the chassis isnt earthed, then you may have oter problems

 

[electricalvillage]

There is no earth rod in use on any of the three vehicles. I asked about this and the chap said that "it isnt required". As this isnt my area I couldnt work out how the RCD would trip given there is no earth. I was taking portable items away for test so I suppose shouldnt be commenting on the vehicle electrics I suppose, however, I was concerned enough to get my 17th edition out when I got back. Still dont fully understand it.

 

[Andy™]

try looking at BS7909 - this site has some information on there

from on board generation, if the genny is TNS and the chassis bonded, then you will have an earth path between live conductor and the chassis. you will not however have a path between live conductor and star point if your outside the vehicle. (unless there is a low resistance to ground between chassis ground - i.e earth rod)

 

[electricalvillage]

Cheers Andy. Give me a building any day of the week!