Delta to Delta Connections

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don_resqcapt19

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Rick,
The secondary current can only go back to the secondary winding. It is magnetically coupled to the primary winding a the primary current will increase based on the turn ratio, but in no way can the secondary current return to the primary circuit.
Don
 

RUWired

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Location
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Ok,So the x4 in the delta -delta gets grounded and carried through to the mcc which provides the fault path. The ground fault on the secondary imposses such a high current on the primary that the primary ocpd trips.This is why only primary ocpd is nessesary.
Rick
 

don_resqcapt19

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Rick,
Ok,So the x4 in the delta -delta gets grounded and carried through to the mcc which provides the fault path. The ground fault on the secondary imposses such a high current on the primary that the primary ocpd trips.This is why only primary ocpd is nessesary.
While you are correct that you only need primary protection for the transformer, as soon as you have a 4 wire system, you need protection for the secondary conductors. The primary protection is permitted to protect the secondary conductors only when you have two wire to two wire transformers or 3 wire delta to 3 wire delta transformers. There are two sets of overcurrent protection rules at work here. The ones in Article 450 for the transformer itself and the ones in Article 240 for the conductors. You have use both sets.
Don
 

iceco1

Member
So if I do not bond X4 then I will need ground detectors? If I do bond X4, the secondary side will be grounded? I guess I'm still a little unclear on the installation. 75kva transformer. Protected at the primary side, also going to fused disconnect on secondary side then to old MCC section.

Tom
 

don_resqcapt19

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So if I do not bond X4 then I will need ground detectors?
yes
If I do bond X4, the secondary side will be grounded?
yes
I guess I'm still a little unclear on the installation. 75kva transformer. Protected at the primary side, also going to fused disconnect on secondary side then to old MCC section.
You have to make sure that the secondary conductors are protected per 240.21(C) and the the secondary grounding is per 250.30.
Don
 

sandsnow

Senior Member
Infinity is correct. This system cannot be grounded so that the voltage will not exceed 150V to ground. My mistake...duh!

Don
Although I would agree that ground detectors are desireable, I don't believe the Code requires them. I could only find them required under ungrounded control xfmr's and high impedance grounded neutral systems. Kind of scary the code lets this happen, that is unless I missed something.

Ungrounded systems should only be installed where qualified persons will maintain the system. My opinion.
 

don_resqcapt19

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Larry,
250.21 ... (4) Other systems that are not required to be grounded in accordance with the requirements of 250.20(B).
Where an alternating-current system is not grounded as permitted in 250.21(1) through (4), ground detectors shall be installed on the system.
 

iceco1

Member
Thanks to everyone that has replied...One last question...What makes this installation a corner grounded Delta? I am not bringing taking one leg and bonding it....I'm taking X4 and bonding it, to give the secondary a reference to ground...
Tom
 

RUWired

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Location
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Tom, This is not a corner grounded xfrmr.Don was saying that if there was a ground fault on one of the secondary conductors,that it would be the same as a corner grounded delta xfrmr.
Rick
 

iceco1

Member
Is a corner grounded transformer factory set up that way, or field installed to a particular application? I guess I'm getting hung up on: #1 What a corner grounded Delta looks like, in a field application. #2 The proper way to ground the secondary side, in my installation.

Tom
 

sandsnow

Senior Member
iceco1 said:
Is a corner grounded transformer factory set up that way, or field installed to a particular application?

A corner grounded is a field connected thing

I guess I'm getting hung up on: #1 What a corner grounded Delta looks like, in a field application.

Looks just like any other XFMR, except X2 (B-phase to some, really doesn't matter except for local convention) is bonded and grounded. Then B-phase is taped white because it is a grounded conductor.

#2 The proper way to ground the secondary side, in my installation.

Bond and ground X4 just like you would Xo. It will be a simple install then.

Tom

Do a little search here for hi-leg delta and you should find some stuff
 
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