If a 3 pole static transfer switch is used to serve a 208/120V load and the static switch is connected on each line side to a transformer, is there a code violation by not switching the neutral at the static switch and bonding the neutrals of the two transformers together at the static switch? Both transformers are connected to a common ground electrode system.
Static Transfer Switches
- Thread starter eh3101
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Re: Static Transfer Switches
What are you calling a static transfer switch?Why would you want to bond 2 transformer neutrals? Would you bond nuetrals from 2 transformers together if they were not going through this transfer switch?
This could let current flow back to a denergized source. Is one of these coming from a generator?
What are you calling a static transfer switch?Why would you want to bond 2 transformer neutrals? Would you bond nuetrals from 2 transformers together if they were not going through this transfer switch?
This could let current flow back to a denergized source. Is one of these coming from a generator?
ron
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Re: Static Transfer Switches
A static transfer switch is a solid state version of an automatic transfer switch. It switches between sources fast (1/4 cycle or so).
There is no code requirement to have a switched neutral. It would be the same as if the sources where normal feeders. The neutrals of the sources (and the load for that matter) would be solidly connected at the transfer switch.
A static transfer switch is a solid state version of an automatic transfer switch. It switches between sources fast (1/4 cycle or so).
There is no code requirement to have a switched neutral. It would be the same as if the sources where normal feeders. The neutrals of the sources (and the load for that matter) would be solidly connected at the transfer switch.
Re: Static Transfer Switches
I basically agree with Ron. This is a slight variation of the "3 pole or 4 pole" transfer switch question we have all seen. Only instead of a generator on the other input, we have a transformer.
You want to make sure you only bond the neutral to ground in one transfomer, and that it is sized correctly for either transformer. I think that would probably mean bonding at the larger transformer if they are different sizes. I'm thinking of the possibility that the neutral and ground to the smaller transformer would be too small for the larger one.
Steve
I basically agree with Ron. This is a slight variation of the "3 pole or 4 pole" transfer switch question we have all seen. Only instead of a generator on the other input, we have a transformer.
You want to make sure you only bond the neutral to ground in one transfomer, and that it is sized correctly for either transformer. I think that would probably mean bonding at the larger transformer if they are different sizes. I'm thinking of the possibility that the neutral and ground to the smaller transformer would be too small for the larger one.
Steve
Re: Static Transfer Switches
This may not have a thing to do with the neutral and grounding methods but I smell another posible proble with the design. I trust that the transfer will be in-phase or you're going to get one heck of bump with an out of phase bump.
This may not have a thing to do with the neutral and grounding methods but I smell another posible proble with the design. I trust that the transfer will be in-phase or you're going to get one heck of bump with an out of phase bump.
Re: Static Transfer Switches
Most applications of static transfer switches are used in computer rooms where swithcing needs to occur quickly (less than 1/4 cycle). Why? Because IEC complient computers use SMPS (Swiched Mode Power Supplies) that use DC voltage. This DC voltage allows the computers to "build up" a tolerance to small power glitches (like a transfer).
The sources providing power to the Static Transfer Switch are normally synchronized with each other through some sort of Sync Cabinet or the nature of their outputs (in the case of UPS units). Some Static Transfer Switches also have the ability to Sync the inputs so when a transfer occurs there is not ony large currents and damage to the loads.
Most applications of static transfer switches are used in computer rooms where swithcing needs to occur quickly (less than 1/4 cycle). Why? Because IEC complient computers use SMPS (Swiched Mode Power Supplies) that use DC voltage. This DC voltage allows the computers to "build up" a tolerance to small power glitches (like a transfer).
The sources providing power to the Static Transfer Switch are normally synchronized with each other through some sort of Sync Cabinet or the nature of their outputs (in the case of UPS units). Some Static Transfer Switches also have the ability to Sync the inputs so when a transfer occurs there is not ony large currents and damage to the loads.
Re: Static Transfer Switches
rich000,
You're correct. That's a good explanation to eh3101 of the right way a transfer should be made. Because that wasn't the question that eh3101 asked he didn't describe the method of transfer was a or what the application was. I wanted to bring this to has attention so that it is not overlooked and then he has another problem.
As an application engineer I have become a bit guarded when I see one part of the story and always point out if they considered what the implications are. I can't count the times when I have gotten the reply of "I never thought of that."
rich000,
You're correct. That's a good explanation to eh3101 of the right way a transfer should be made. Because that wasn't the question that eh3101 asked he didn't describe the method of transfer was a or what the application was. I wanted to bring this to has attention so that it is not overlooked and then he has another problem.
As an application engineer I have become a bit guarded when I see one part of the story and always point out if they considered what the implications are. I can't count the times when I have gotten the reply of "I never thought of that."
Re: Static Transfer Switches
rich000,
I'm well aware of the application as I sold ATSs for 18 years which included breaker types to the 4000a stored energy type power breakers with "inphase" and "closed transition" transfer schemes as well as contactor type with all of the bells and whistles. That's why I brought the point up, thus my comment to an "inphase transfer." It's the "Oh, I forgot to tell you" or "Gee, I never considered that" with regard to what they are really trying to do.
ASCO is one old and respected manufacturer that uses a very fast transfer technique that can cause grief if the transfer is made into another source which is out of phase. One of ASCOs good points is it's fast transfer if done correctly, that is made within 1/4cyc.
Dave
rich000,
I'm well aware of the application as I sold ATSs for 18 years which included breaker types to the 4000a stored energy type power breakers with "inphase" and "closed transition" transfer schemes as well as contactor type with all of the bells and whistles. That's why I brought the point up, thus my comment to an "inphase transfer." It's the "Oh, I forgot to tell you" or "Gee, I never considered that" with regard to what they are really trying to do.
ASCO is one old and respected manufacturer that uses a very fast transfer technique that can cause grief if the transfer is made into another source which is out of phase. One of ASCOs good points is it's fast transfer if done correctly, that is made within 1/4cyc.
Dave
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