Using a contactor with emergency stop button

[Pensdown]

That is incorrect, data centre epo's normally operate via n/o contacts, the circuits are fault monitored and sometimes duplicated. The logic is to avoid the risk of a break in the circuit causing an expensive shutdown. Therefore any control circuits that could effect IT operations should be in a quiescent (de-energised) state.

Emergency switching should "interrupt" the appropriate supply (537.4.1.3) So if the supply is via a contactor the coil circuit must be "interrupted" i.e. N/c.

I hope we're not getting confused between a single point of shutdown for whatever reason and emergency switching.

 

[ericmark]

Not that it really matters here but note to Pensdown the Guardmaster range of emergency stop relays and switches do include a double switch unit which has one N/O and one N/C switch designed so should something short out the cable it will still shut down. I will agree a N/O is not used on it's own but they are used.

 

[holmslaw]

..

 

[Pensdown]

As you’re aware, the main reason EPO's are no longer used in data centers is for that reason. They are a very unreliable single point of failure. Of course a de-energised system would be better but it then becomes un-reliable as an emergency stop circuit.

But unless someone has specifically identified a risk as far as I’m aware, most EPO's in data centers do not have meet either BS EN954 or IEC 61508.

Here is a useful document which lists the categorys and the circuits which comply - all using fail safe N/c contacts.

Anyway, we'll have to agree to disagree on this one.

http://www.hs-compliance.com/uploaded/documents/THE EMERGENCY STOP.pdf