Another earthing question

[adrianh]

I went to a photolab yesterday and spoke at great length to the owner. He told me about a problem he's having with one of his R1.2mil procesing machines that is turning into long drawn out saga. The machine is insured and is being repaired by qualified technicians and nobody can fault their service.

Anyhow, this is what happened; the machine was running and the maid was doing her daily vacuuming rounds. The vacuum cleaner power cord was suffed and the live line touched the metal body of the machine....the machine got fried.

Ok, so my questions are as follows;
1. Shouldn't eatch leakage have protected the machine or was the transfer of current so large and fast that bythe time the earth leak breaker responed it was too late.
2. Why would touching the bodyof the machine fry the electronics, provided of course that the body of the machine has no electrical contact with the machine other than being earthed?

[Justloadit]

Theoretically, the ELCB should have tripped, and no further damaged should have been caused.

However when a running machine is abruptly disconnected from the supply, and if there are huge inductive loads in the running machine, then theoretically an extremely high pulse of power surge is generated at the moment of the power interrupted in the mains supply lines, which could cause some components to fail. Invariably the PSU takes the knock and fails, but the damage may not only be limited to the PSU, under these conditions.

This the one reason I always recommend a UPS for any electronic controlled system. The UPS gives you time to either correct the mains failure, or to shut the machine down in a controlled manner.

[adrianh]

Fair enough but machines are designed to withstand power supply trips. Those machines work all day every day and are bound to have experienced many power supply trips before.

[AndyD]

Some machines have a communications system that is earth referenced, a large fault current down the earth could possibly blow coms cards etc. Generally though I wouldn't expect an earth fault to cause damage to the machine and it's impossible to say why it did without a lot more details about the machine itself and what exactly was damaged.

[Slow Blow]

If the machine was connected to a "clean earth" system and this system is connected back into the general building earthing system and there is an earth/neutral disconnection fault to the ground spike system then theoretically you could have had a return surge through the neutral, hence a fried PSU.

Then again how many sparkies understand the term "clean earth", there is no way out of this one methinks.

[Dave A]

Then again how many sparkies understand the term "clean earth"

For those that don't, a little light reading here and here might help.

The upshot of it all seems to be - have the chassis bonded to the power earth, and the electronics bonded to the clean earth. This way in the o.p situation the energy would all flow through the power earth, (giving your electronics a better fighting chance).

[ians]

I haven't seen a "clean earth" being used since year dot. even the dedicated plugs no longer have a separate earth. Got involve din some lightning protecting projects at one stage, went to a few seminars and this clean earth separate earth system is not the way to go. What seems to come out was people turning more towards the all in one earth system, where everything is bought to one point and grounded correctly. Just a note I haven't felt the urge to waste my time at seminars etc in while , things might have changed again.