DC battery back up units

[Bunsen]

All,

I have had a battery back up unit installed for warehouse lighting for the emergency lighting.
It has stopped working and the investigation was found to be a short circuit that affected the unit.
However after being informed by the designer that both the positive and negative connections would've needed to touch the casing (earth) but unless I am having a mind blank only the positive would require to touch the casing (earth) to take a chunk out of the casing and short? Under the other circumstance when it was being installed if the connections were on the battery and one of them touched the casing this would also cause a short?

As I wasn't around for the installation and have been landed with this I would appreciate all the input available.

I feel as if I am having a mind blank and hopefully I am not.

regards,

 

[Deleted member 26818]

Why would you think only the positive would need to touch to cause a short circuit?

 

[Richard Burns]

Both positive and negative of the battery are needed to connect the battery via the metal casing. There is no connection to earth for the battery on either pole so there is not a circuit when one pole of the battery touches an earthed piece of metal.
If the battery system had one pole earthed to the LV system earthing (not a good idea) then the other pole touching and earthed conductor would cause a short.