I was looking at transfer switches that use 3 pole breakers to switch the neutral on a typical residential service. They mentioned that the neutral will be first to make contact and last to break, otherwise you could get a voltage spike (just like a loose/broken neutral) for a short time period as the breaker opens/closes. I don't know if this is a real or theoretical issue. Also don't know if they just used a typical 3 pole breaker for this or it was some special one where the neutral pole was guaranteed to close first and open last.
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Three Phase Breaker Single Phase Power
- Thread starter A-A-Ron
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That is how manufacturer's representative explained it when I asked about a product they are selling. The neutral pole should make first and then breaks last. If this is a normal three-pole breaker, I have doubts if the pole with the neutral connection would function as desired.I was looking at transfer switches that use 3 pole breakers to switch the neutral on a typical residential service. They mentioned that the neutral will be first to make contact and last to break, otherwise you could get a voltage spike (just like a loose/broken neutral) for a short time period as the breaker opens/closes. I don't know if this is a real or theoretical issue. Also don't know if they just used a typical 3 pole breaker for this or it was some special one where the neutral pole was guaranteed to close first and open last.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
That is how manufacturer's representative explained it when I asked about a product they are selling. The neutral pole should make first and then breaks last. If this is a normal three-pole breaker, I have doubts if the pole with the neutral connection would function as desired.
First make/last break is important for transferring between live sources, it is immaterial for a simple on-off operation such as described by the OP.
Why? The transfer switch goes from source A, to off, to source B which is effectively two separate on-off type operations. The problem is when removing or energizing a source and the neutral isn't behind the ungrounded conductors -- you could get a "broken neutral" condition for some fraction of a second as the poles are closing or opening.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Why?
It has to do with the speed that a transfer switch operates at. Many automatic transfer switches do not have an off position.
Your scenario could occur anytime a 3 phase 4 wire plug is removed or inserted under load. How many of them have you seen with early make - late break neutral connections?
Yes, but at least that would be just a single device being affected, and perhaps it would be designed to accommodate that possibility. A transfer switch is doing an entire feeder -- lots of things on that especially line to neutral loads. I'm thinking the time period would be quite short, but could see think like surge suppressors being killed over time with each power transfer possibly making spikes over the SPD rating for a half a power cycle or so.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
... I'm thinking the time period would be quite short ...
The short time period of the transfer is the problem. Manually operated double throw switches often do not have overlapping neutrals because there is usually an appreciable delay that allows the system to stabilize before being re-energized. In automatic transfer schemes this is often called an imposed neutral.
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