NFPA 99 and Medium-Voltage-Transfer-Switches in Hospitals

[mdomran]

Hi Forum

My office is currently undertaking the electrical design of a new hospital building in Saudi Arabia - The calculated maximum demand for this facility is quite large, in the tune of a few Megawatts.

The building is served at 13.8kV, with on-site diesel generation also at 13.8kV.

The transfer-switching arrangement is proposed to be installed upstream of the transformers (i.e. using MV Transfer Switches), with dedicated transformers and generators each for the Life Safety, Critical and Equipment branches.

Now NFPA 99 6.4.2.2.1.4 states
?
The number of transfer switches to be used shall be based upon reliability, design, and load considerations.
(A) Each branch of the essential electrical system shall have one or more transfer switches.
(B) One transfer switch shall be permitted to serve one or more branches in a facility with a continuous load on the switch of 150 kVA (120 kW) or less.
?
Does this Clause (B) above, imply that MV ATS? are not to be used, and only LV ATS? are permitted?

Would much appreciate some feedback on this.
An option we are considering is to have the Authority Having Jurisdiction, overrule this clause if LV ATS' are in fact the requirement.

 

[roger]

(B) simply means if you have a small facility you can use a single switch, see Figure No.2 NEC 517.30.

Roger

 

[JoeStillman]

Could the problem be the UL listing (or lack thereof) for MV transfer switches?

Also, your generators don't protect you from the failure of a transformer - not that the code requires it, but is it good practice?

 

[steve66]

Could the problem be the UL listing (or lack thereof) for MV transfer switches?

Also, your generators don't protect you from the failure of a transformer - not that the code requires it, but is it good practice?

Yes, I would consider that good practice. Not that one has to build redundancy into every single transformer, but I would say having the entire Life Safety branch dependant on a single MV transformer would be a very bad design.

This would also apply to the critical branch, and the very important parts of the equipment branch (including medical air compressors and vacuum pumps, etc.)

As Roger mentioned, this isn't from part "B", but from the 1st sentance of teh code paragraph quoted.

In general, the most reliable designs put the transfer switches as close to the load as possible. Its the engineers job to balance that fact with economics and other building factors.

Consider that most interuptions of power are not from utility outtages, but from problems within the hospital. These include tripped breakers, construction and remodeling, overloaded circuits, etc. The farther upstream the transfer switches are, the less able they are to deliver power in spite of these interuptions.

 

[mbrooke]

I wouldn't fool around with medium voltage transfer switches for life safety. Yes MV switch gear is held to higher standards over LV, but from most life system Ive seen multiple low voltage switches were used. Granted there some hospitals that have a massive generator plant that uses SCADA switchgear for an ATS to power multiple facilities ina complex but those frequently have an additional gen set (s) for the ultra critical branch circuits. Artice 700, 701 and 702 loads are kept separate as well (legal, critical and life safty). In an emergency its easier and much safer to work on/bypass a low volt ATS then MV breakers used for load transfer applications, and good practice breaks up the emergency load onto multiple transfer switches as close to the load as possible often with an area getting source from at least 2 ATS (Intensive care, OR, Hall lights are fed from 2 separate panels on 2 separate ATS. (some specs going as far from 2 separate emergency sources but thats another option)

I would never really on a single MV power transformer, if it fails good luck getting the power on, and good luck finding a replacement soon enough.

However so far I don't see the code requirement asking it be MV or LV.

 

[mbrooke]

Just to add the 150kva rule applies only if the load is no larger of the combined critical and life safety system which means the 2 can be together but if the sum of the two exceed this they each must have there own ATS.