EBME Forums Forums Maintenance Technical Assistance mixing isolated and grounded power supply in OR

All class I equipment will be connected to ground (earth) whether you connect to an isolated (medical IT supply) or grounded system (TN-S etc.). This is the fundamental of the safety of the equipment. This is one common misunderstanding of an isolated supply; the earth pin of the socket-outlet still connects to earth!

If you had no earth (ground) on a class I equipment then all would be fine in the case of a single fault to earth but if another device on the same isolated supply then failed, before you disconnected the first, then the chassis of all devices would become live and the protective device (fuse) would never blow!

The use of equipotential bonding leads is optional (see IEC 60601-1) but are designed to bring equipment earths into an equipotential state. In a good electrical installation they would have little effect but could help mitigate a possible open earth of the equipment protective conductor (for example when performing CVC dialysis using only a machine with a B type applied parts rating).

Yes, all "earth" pins on the outlets of an isolated system are ganged together at the "system earth bar" (probably inside the ISP distribution panel) ... and this will be bonded to "building earth"* just like the "earth" of a normally grounded outlet. This is the route that leakage current takes when Class 1 equipment has a decent protective conductor in its mains power cable.

I like to keep things simple (and often resort to a sketched diagram to assist my tired brain) ... and would say that the essential point to be borne in mind is that in a normal grounded system the Neutral line is tied to earth, whilst with an isolated system it is not.

In other words, if you stuck your meter across the neutral and earth pins at a normally grounded outlet, you would expect a few volts only (tending to zero).

Trying the same thing at an isolated outlet, you would get something like half the supply value (110 volts or so in the UK, as mentioned earlier).

Whether mixing up two systems like this within the same room - and when considering all the possible fault conditions that can occur in mains powered equipment - is a wise course of action (or, as I have already suggested, an "accident waiting to happen") I shall now leave to the Gentle Reader!

* That's "proper earth" - the one that sees a copper rod rammed into the ground somewhere back along the line.