Automatic Transfer Switch

Status
Not open for further replies.
J

jrainwater1

Member
I have a customer with an automatic transfer switch. The switch is a 3-phase, 3-pole, 3-wire system. The voltages were measured as follows:
1.Pole to Pole voltage: A to B = 230 vac, A to C = 230 vac, B to C = 0 vac
2. Each pole to ground voltage. A to Gr. = 115 vac, B to Gr. = 115 vac, C to Gr. = 115 vac
3. For the sake of clarity, we will call the pole to the left as you are facing the contactor phase A, the middle pole phase B, and the far right pole phase C.

Why is the voltage from B to C zero?

I want to replace the automatic transfer switch but I have never seen a configuration like this before. Can someone tell me if they have or what they suspect is going on?

Thank you,

Jerry
 
jrainwater1 said:
The voltages were measured as follows:
1.Pole to Pole voltage: A to B = 230 vac, A to C = 230 vac, B to C = 0 vac
2. Each pole to ground voltage. A to Gr. = 115 vac, B to Gr. = 115 vac, C to Gr. = 115 vac
Click to expand...
What kind of meter are you using? These numbers should not be possible from one service.
 
You may have a three phase open-delta system utilizing 2 x two bushing transformers with a ground on the secondary side incorrectly placed. It may not be the case, but it would be a place to start.

If not, what is the configuration of the incoming utility transformers?
 
Here's one possibility; suppose that at one time this xfer switch was fed by a 240/120 3PH 4W delta, with the high leg either B or C. For whatever reason, possibly the installation of a single phase only gen, the normal side was converted to single phase and B and C were jumpered together so all 3 busses downstream would be hot. Obviously, there are presently no 3 phase loads, and the only 240V loads are across A to B, or A to C.

It's also possible (scary) that the jumper is installed downstream from the xfer switch, but almost certainly there's a jumper, at some location, between B and C. It could even be a 2 pole breaker with both poles tied together.
 
Find the service or source, and make sure it is three phase.

And what is the switch feeding? Three phase loads, or single phase loads?

Steve
 
Status
Not open for further replies.
Top