bonding transformers to building

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clcsmooth

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I work in an industrial factory we have an r&d department the have constructed a new machine that runs on 480/277 volts the have a 30 kva transformer to step down voltage to run 120 volt applications the transformer is on a cart with caters does this transformer need to bonded to building metal
 
clcsmooth said:
I work in an industrial factory we have an r&d department the have constructed a new machine that runs on 480/277 volts the have a 30 kva transformer to step down voltage to run 120 volt applications the transformer is on a cart with caters does this transformer need to bonded to building metal
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if it is an SDS, it has to be bonded to the closest grounding electrode. That may or may not be the building structure. you can avoid this bond by arranging it so it is not an SDS.
 
Welcome, clcsmooth. :thumbsup:

SDS = separately derived system. Your typical step-up and step-down transformer applications have isolated primary and secondary.

If this transformer is on a cart, that seems to indicate it is portable. How is it connected to the supply?
 
petersonra said:
what does it accomplish if it is not on a cart?

why is it Ok not to do so if it is a 1KVA or smaller xfmr?

someone decided it has to be this way.
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That's my question, on a cart or on the 20th floor of a building what does it accomplish by connecting it to building steel?
 
Smart $ said:
Welcome, clcsmooth. :thumbsup:

SDS = separately derived system. Your typical step-up and step-down transformer applications have isolated primary and secondary.

If this transformer is on a cart, that seems to indicate it is portable. How is it connected to the supply?
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Is there any requirement that they have to be isolated from each other? I can't require ever having seen a 3 phase step down xfmr that wasn't but I don't recall there is any rule that says if you have a 480/277 primary and 208-120 secondary that says you can't connect the neutral on the secondary to the neutral on the primary. You would just have to bring in all four wires on the primary side. that could be a real hassle some places where the neutral isn't run past the first panelboard. but a lot of places use 277V lighting so they would have run it out to most PBs in all likelihood.

There are a lot of 3 phase step up/down systems that only use two transformers and they are not SDS.
 
petersonra said:
Is there any requirement that they have to be isolated from each other? I can't require ever having seen a 3 phase step down xfmr that wasn't but I don't recall there is any rule that says if you have a 480/277 primary and 208-120 secondary that says you can't connect the neutral on the secondary to the neutral on the primary. You would just have to bring in all four wires on the primary side. that could be a real hassle some places where the neutral isn't run past the first panelboard. but a lot of places use 277V lighting so they would have run it out to most PBs in all likelihood.
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There is no requirement primary and secondary be isolated.

There are a lot of 3 phase step up/down systems that only use two transformers and they are not SDS.
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Can you be more specific? Are you referring to buck/boost autotransformer applications?

Now that the subject has resurfaced, I was wondering if a non-SDS configuration of an isolation transformer could/should/would be considered an autotransformer configuration and thus have to follow autotransformer rules...???
 
I was indeed thinking of Auto Transformer applications. I don't think it's an autotransformer just because it's not isolated it's an auto Transformer if it's an auto Transformer.
 
petersonra said:
I was indeed thinking of Auto Transformer applications. I don't think it's an autotransformer just because it's not isolated it's an auto Transformer if it's an auto Transformer.
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Wow... never would have thought of that in a million years. :slaphead:
 
Smart $ said:
....Now that the subject has resurfaced, I was wondering if a non-SDS configuration of an isolation transformer could/should/would be considered an autotransformer configuration and thus have to follow autotransformer rules...???
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petersonra said:
I was indeed thinking of Auto Transformer applications. I don't think it's an autotransformer just because it's not isolated it's an auto Transformer if it's an auto Transformer.
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Smart $ said:
Wow... never would have thought of that in a million years. :slaphead:
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Well it's either an auto transformer or it's a SDS, no? Except... do you have an auto transformer if you are tying the primary and secondary neutrals together?
 
ActionDave said:
Well it's either an auto transformer or it's a SDS, no? Except... do you have an auto transformer if you are tying the primary and secondary neutrals together?
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I think it is only an autotransformer if it is one. An autotransformer is not made by tying the primary and secondary neutrals together, because pretty much by definition an autotransformer does not have a primary and a secondary. It is effectively all one winding.

The code does not seem to have a definition of autotransformer.
 
petersonra said:
I think it is only an autotransformer if it is one. An autotransformer is not made by tying the primary and secondary neutrals together, because pretty much by definition an autotransformer does not have a primary and a secondary. It is effectively all one winding.

The code does not seem to have a definition of autotransformer.
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A buck-boost is an isolation transformer configured by winding connection to operate as a buck or boost autotransformer. It technically has two windings. Connecting it to function as an autotransformer does not have to comply with Code requirements for autotransformers?
 
Smart $ said:
A buck-boost is an isolation transformer configured by winding connection to operate as a buck or boost autotransformer. It technically has two windings. Connecting it to function as an autotransformer does not have to comply with Code requirements for autotransformers?
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Read what I wrote.
pretty much by definition an autotransformer does not have a primary and a secondary. It is effectively all one winding.
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QA buck boost transformer is an autotransformer becasue the windings are all in series so it is effectively a single winding.
 
petersonra said:
Read what I wrote.


QA buck boost transformer is an autotransformer becasue the windings are all in series so it is effectively a single winding.
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That's just it. Define "effectively a single winding". With the primary and secondary neutrals tied together, the primary and secondary windings effectively become one winding, do they not?
 
Smart $ said:
That's just it. Define "effectively a single winding". With the primary and secondary neutrals tied together, the primary and secondary windings effectively become one winding, do they not?
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After pondering this for a day I come to the same conclusion. We are given two choices, SDS and auto transformer. If the windings are tied together it's not an SDS, so it must be an auto transformer.
 
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