Keeping the Lights On Response

Q. I liked your article Keeping the lights on in the recent HFM Magazine. I was hoping you could elaborate in more detail the section on parallel generators used for level 1 EPSS systems. We have a parallel generator system with load shed/delayed connection on the newest equipment branch ATS to keep the load below the rating of one 1500kw generator. I didn't understand how the tiebreaker system works to correct this problem. Please send a simple drawing to help explain. Thanks.  E.F.

A. When you have a paralleled system with multiple generators the tie-breaker on the paralleling gear is normally open. Essential Level 1 loads (life safety and critical) can be fed on both sides of the tie.

When the generators are called to start, one generator can be synchronized to the 'A' side of the bus and another to the 'B' side of the bus within 10 seconds. The tie will then close and the remaining generators will be attached to the bus - which you now can think of as continuous, as all generators are now all on the same bus.

Without the tie the paralleling gear would only be able to accept the smallest genset in the lineup, to be synchronized and on-line within 10 seconds per code. Having the tie allows you to double the level one loads and not be limited to the single generator set. Think of it as two separate busses before the tie automatically closes after the one genset connects to each side of the tie, and then one bus after the tie is closed.

Let me know if this helps you to understand and if I can be of further help.

Vince M. Rea, P.E., LEED AP
Associate, Senior Electrical Engineer
Healthcare Division
TLC Engineering for Architecture
Vince.rea@tlc-eng.com