Neutral bond in transformer

[teco]

Hello,

Can someone tell me why one should not ground or bond the neutral (X0) on a Delta to Wye transformer if the neutral is not going be used?

Thanks

 

[texie]

With very few exceptions, you must bond the X0 as the separately derived system must be grounded. Without the X0 bond you would have no fault return path for ground faults.

 

[teco]

With very few exceptions, you must bond the X0 as the separately derived system must be grounded. Without the X0 bond you would have no fault return path for ground faults.

One of my electrician's said he had an engineer and an inspector once ask him not to bond the X0 terminal to the transformer enclosure with the grounding conductor if the secondary load does not utilize a neutral connection. This again came up in his master's class last night. When ask no one could tell him why, just not to do it. I'd like to know why?

 

[roger]

I would say there are some people confusing back feeding a Wye/Delta verses a Delta/Wye that has no loads needing a neutral. In the latter the XO must be grounded and bonded to provide fault clearing, so those who say it should not be done, are wrong.

Roger

 

[mike_kilroy]

u must knokw what is being powered...

u must knokw what is being powered...

Hello,

Can someone tell me why one should not ground or bond the neutral (X0) on a Delta to Wye transformer if the neutral is not going be used?

Thanks

in my opinion, this is not a simple one answer is right for all situations question..... whether to ground neutral or not can depend on the equipment being powered by the wye source.

some electronic equipment like afd,vfd, servo drives, REQUIRE a grounded nuetral for either basic operation or for voltage spike protection; this can be expanded on why but prob not necessary here right now.

as an opposite, there are still some drives in use that ground their dc bus minus: so if you ground your nuetral, you will have direct line to ground short thru transformer and will blow that drives input rectifiers.

so to decide the proper answer, one must look at what is being powered and make an educated decision. If there is NOTHING requiring one way or the other, grounding it is the best decision as it provides a low Z path to ground for all 3 phases and that can help suppress noise as well as limit the amplitude of any spikes from such things as lightning strikes nearby. Of course as previously stated, it does also provide a good safety ground which is a good thing.

 

[texie]

One of my electrician's said he had an engineer and an inspector once ask him not to bond the X0 terminal to the transformer enclosure with the grounding conductor if the secondary load does not utilize a neutral connection. This again came up in his master's class last night. When ask no one could tell him why, just not to do it. I'd like to know why?

Tell 'em that without the X0 bond that grounding conductor you are refering to would have no fault return path to make the OCPD operate. There are rare cases to not bond the X0, but that is a whole 'nother discussion with different rules. Be aware that this is a serious issue, with serious safety implications.
Maybe some of the other guys here will chime in with some of the good graphics to illustrate why you MUST always bond X0.

 

[mike_kilroy]

i should temper my reply (#5) above with I am not a high power or safety engineer, but rather a servo electrical engineer who has to contend with individual typically 10-500kva transformers supplying typically a single machine; there are definite cases where X0 is still not tied, but this is not a general power source running around a plant so may be totally different case than the OP asked for.

 

[roger]

i should temper my reply (#5) above with I am not a high power or safety engineer, but rather a servo electrical engineer who has to contend with individual typically 10-500kva transformers

Quite a bit different than the conversation at hand isn't it?


Roger

 

[mike_kilroy]

not sure; application was not listed..... may be a question on a single machine.

 

[templdl]

Hello,

Can someone tell me why one should not ground or bond the neutral (X0) on a Delta to Wye transformer if the neutral is not going be used?

Thanks

Understanding that you are referring to a step down transformer, yes, you would bond the X0 to the transformer frame, and since you have derived a new system you would ground that point appropriately.
You have indicated that the neutral will not be need because you will not have any neutral current which is fine. But remember that it may be inconvenient to pull a neutral should someone decide that they want to add some 1ph L-N loads in the future.
But you still must run an EGC.

 

[stew]

one of the most dangerous things not to do on any separatly derived system is to leave this bond out. IMHO and many others i am sure as well. How can you clear a fault without the bond to neutral.???? wait ten minutes?

 

[kwired]

How can you clear a fault without the bond to neutral.????

Ground a phase. Just don't ground a phase and the neutral.

 

[stew]

@kwire in most cases grounding a phase is not done is it? If so when and why would one do that instaed of bonding the neutral. I assume you meant bond a phase to ground correct?

 

[texie]

Ground a phase. Just don't ground a phase and the neutral.

An intentionally grounded phase on a wye secondary?
I hope we don't stray off the path and confuse the OP further than he was when he posted.

 

[qcroanoke]

Ground a phase. Just don't ground a phase and the neutral.

Did that once on a 220 2 wire (phase to neutral) 50 hertz generator application at the engineers direction.
I made him turn it on!! Couldn't be convinced that it wouldn't let the magic smoke out!

 

[templdl]

An intentionally grounded phase on a wye secondary?
I hope we don't stray off the path and confuse the OP further than he was when he posted.

Precisely, this is my concern as this may be straying away form the initial intent of the OP. As a former application and sales engineer I have been very cautious about giving a client a loaded gun that is addressing an issue based upon assumptions and incomplete information that is provided to me such that my answer can be applied incorrectly. In this case a simple 3ph ?Y? transformer (with no ?s? after transformer which means a single transformer) being applied with 1ph loads is the issue.

 

[teco]

Okay, Here is the application. The customer supplied a 500 KVA dry type transformer. 480 volt delta primary, 120/208 wye secondary. Our job was to connect all the raceways and wiring. The secondary load is a very high tech tool for the semi conductor industry. The transformer secondary is to feed a 1200 amp main breaker with 208 volts 3 phase plus the equipment ground. The customer already owned the transformer (new) and requested that it be used for the job.
One of my new electrician's on the job brought up that on a previous job, when using a wye secondary transformer, where no neutral is needed at the load, he was asked by the engineer designing the job not to ground or bond the X0 terminal on the transformer.
I did not do this and we grounded and bonded the transformer as we always do. I stated if we did not do this, there would be no potential to ground on the secondary and would be a dangerous thing to do.
It came up again weeks after the install in the master class the same electrician is enrolled in. The teacher, who is also an inspector said the transformer secondary should not be bonded or grounded if the neutral is not used. I asked my electrician to ask the teacher why? He said he did, but got no answer. The guy said thats just the way you should do it.

I figured someone on the forum may know some reason why these guys would want this done this way. I've never heard of it in my 27 years of business.

 

[cadpoint]

one of the most dangerous things not to do on any separatly derived system is to leave this bond out. IMHO and many others i am sure as well....

For me, the hardest thing for me to remember is the phrase; a transformer is a seperately derived system... :dunce:

: ) one has to do something!

 

[kwired]

Okay, Here is the application. The customer supplied a 500 KVA dry type transformer. 480 volt delta primary, 120/208 wye secondary. Our job was to connect all the raceways and wiring. The secondary load is a very high tech tool for the semi conductor industry. The transformer secondary is to feed a 1200 amp main breaker with 208 volts 3 phase plus the equipment ground. The customer already owned the transformer (new) and requested that it be used for the job.
One of my new electrician's on the job brought up that on a previous job, when using a wye secondary transformer, where no neutral is needed at the load, he was asked by the engineer designing the job not to ground or bond the X0 terminal on the transformer.
I did not do this and we grounded and bonded the transformer as we always do. I stated if we did not do this, there would be no potential to ground on the secondary and would be a dangerous thing to do.
It came up again weeks after the install in the master class the same electrician is enrolled in. The teacher, who is also an inspector said the transformer secondary should not be bonded or grounded if the neutral is not used. I asked my electrician to ask the teacher why? He said he did, but got no answer. The guy said thats just the way you should do it.

I figured someone on the forum may know some reason why these guys would want this done this way. I've never heard of it in my 27 years of business.

If you are using the neutral you must ground it. If you don't bond anything you have an ungrounded system. You do however need to use a ground detection system and still need to bond all non current carrying metal enclosures, raceways, etc. with the usual equipment grounding conductor and install a grounding electrode system.

Grounding a phase on a wye secondary is probably not too common but could be done. One example of where you may do this is if you need to replace a transformer that was originally a corner grounded delta, but other equipment is not being changed.

 

[templdl]

You have a separately derived system whether your "load" requires or neutral or not.
The X0 in bonded to the enclosure and you ground the X0 as directed by the NEC. The line conductors and a EGC is pulled to the load. Should there be a L-G fault the fault current will flow back to the X0 causing the OCPD to trip. Since the secondary is a 208Y/120 any L-G ground fault will be 120v regardless if there is a neutral pulled to a load or not. To even consider grounding a phase conductor in leu of the X0 does not make sence. If one elects to ground one of the 208v lines a ground fault will now be 208v. What anvantage is that? Is that better than 120v? When there is no choice other than grounding a phase such as corner grounding a 240 or 480v delta I find it unreasonable to even consider grounding one of the phases of a wye. One of the guys said "Just because you can doesn't mean you should."