Can closed transition ATS be used on an emergency system? Or does 700.5(A) "Transfer equipment shall be designed and installed to prevent the inadvertent interconnection of normal and emergency sources of supply in any operation of the transfer equipment." prevent this?
Closed Transition on NEC 700 Systems
- Thread starter jbt260
- Start date
- Location
- Illinois
- Occupation
- retired electrician
While a closed transition transfer switch does connect both the normal and emergency source, that interconnection is not inadvertent. It is common to use a closed transition switch in some applications to avoid any loss of power when the generator is tested.
ModbusMan
Senior Member
- Location
- Cleveland, OH
- Occupation
- Building Automation Engineer
I would be concerned about how to recover if something happens mid-transition... Perfect real-life example:
I was barely three months with my current employer, and we were doing a test of the open-transition medium voltage ATS in the back electric yard. It hadn't been exercised in years, but since one side was de-energized, we didn't expect any sort of issues while the building ran on generator and the high voltage guys did their thing on the side that had been carrying the load all along. So far, so good, right? Here's where things went south... after work was completed and the building back to commercial power, the low-voltage ATS units inside, which were programmed for closed-transition, started their commit timers. One switch made the cut from generator to commercial just fine. The other, however, did not, and got bound up with both sources connected. Five minutes passed, and the first generator shut down. So did the other, but it didn't. It was being back-fed by the utility, and continued this way for about an hour as we tried to figure out why it sounded like it was still running. We even hit the EPO to no effect. Eventually we got to calling Caterpillar, and the very first thing out of their tech's mouth over the phone, before he even got on-site was "OPEN THE BREAKERS!!!" So we did, and immediately the thing stopped spinning. Thankfully no damage was done, but the very first thing we did the next morning was change those 480v switches to open-transition mode.
I was barely three months with my current employer, and we were doing a test of the open-transition medium voltage ATS in the back electric yard. It hadn't been exercised in years, but since one side was de-energized, we didn't expect any sort of issues while the building ran on generator and the high voltage guys did their thing on the side that had been carrying the load all along. So far, so good, right? Here's where things went south... after work was completed and the building back to commercial power, the low-voltage ATS units inside, which were programmed for closed-transition, started their commit timers. One switch made the cut from generator to commercial just fine. The other, however, did not, and got bound up with both sources connected. Five minutes passed, and the first generator shut down. So did the other, but it didn't. It was being back-fed by the utility, and continued this way for about an hour as we tried to figure out why it sounded like it was still running. We even hit the EPO to no effect. Eventually we got to calling Caterpillar, and the very first thing out of their tech's mouth over the phone, before he even got on-site was "OPEN THE BREAKERS!!!" So we did, and immediately the thing stopped spinning. Thankfully no damage was done, but the very first thing we did the next morning was change those 480v switches to open-transition mode.