kiwi, Darned good point mate. Let's see what we have here. A Hospital would have a Double Redundancy system to ensure that there is no break of supply under any circumstances, not even for a nano-second. Medical systems are the cream of Electrical system. As in the Tech's that work on them are trained for that work. Kieran, if you want the email of the local Hospital Tech, who is also an Electrician, e-mail me at the above address. He would be the guy to talk to. Hope this helps.
#143925 - 09/24/0504:36 AMRe: Auto transfer switch in hospital
Hi guys, long time no post ("darn, he's back" I hear some say!).
Let me tell you of a fault I investigated recently; The gen AND panel had N-E bonds. The circulating currents throughout the installation were enormous! (the main current carrying conductors could carry up to 1000A each).
The solution was a single N-E bond in the panel and all Neutral and Earths met at this point.
However, the gen can also run in "augment" mode (supply the national grid with extra juice to stop the voltage & frequency falling too low) but this causes high Neutral currents (harmonics etc.). So in this mode the Neutral to the gen must be switched so as to stop high Neutral currents in the gen.
However, this raises another nasty. If you have a Neutral switch fault (open circuit) while in back-up supply mode, then the voltages that can appear L-N throughout the phases could be anything from subdued to almost 400V (taking 230V norm), and there situations where all three phases can suffer high voltages in this mode (I'm not going to go into that explanation now).
Does this make sense?
#143931 - 12/15/0506:39 AMRe: Auto transfer switch in hospital
Here in Auckland we synchronise generators in on the grid to do maintenance on the 11 kV ring mains units RMU's to have a no break situation for the customers.
This can be anything from 200 to 1000 kVA.
The neutral and earthcables from the generator are connected to the transformer Neutral / Earth bars. The 3 phases are switched through an isolator plus motor operated contactor arrangement via synchronoscope.
Only the phases are switched. The Neutral not.
The product of rotation, excitation and flux produces electricty.
#143932 - 12/16/0504:39 AMRe: Auto transfer switch in hospital
Thanks Rodalco. After I posted this question I did a bit of reading on this subject and you are right. A seperately derived generator does not need a switched Neutral at the transfer switch if it is connected to a MEN system appropriately. I.E starpoint and metalwork of generator connected to Earth at Main Switcboard
Our European cousins however do things differently and have 4 pole transfer switches.
Thanks Y'all for your help on this. I couldn't get advice like that if I paid for it. Long Live ECN ! !