Transformer tap terminating on instantaneous breaker?

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CobyRupert

Member
Location
NY
Similar to a recent post (but different!)

In my situation I have a 500 kVA, 480V transformer, less than 10' feet away and supplying a 600 amp (frame) instantaneous only (electronic!) breaker that is (let's assume) part of a listed combination controller. (Actually its a VFD). The transformer has NO secondary protection.

What I think is interesting (significant) is that the (electronic!) instantaneous breaker of the starter has an adjustment setting of "1" (i.e. a vertical line at 600amps on the current-time curve). My understanding is that being electronic it's not relying on magnetic forces of an "instantaneous" current.

Can I use this setting of "1" (600amps) to protect my 620amps worth of transformer secondary tap conductors? If set to "1", doesn't this provide the same protection as a electronic breaker with long time, short time, inverse time capabilities (i.e a breaker analogous to a thermal-magnetic breaker)? Is 240.21.(C)(2) fulfilled?​
 

templdl

Senior Member
Location
Wisconsin
Similar to a recent post (but different!)

In my situation I have a 500 kVA, 480V transformer, less than 10' feet away and supplying a 600 amp (frame) instantaneous only (electronic!) breaker that is (let's assume) part of a listed combination controller. (Actually its a VFD). The transformer has NO secondary protection.

What I think is interesting (significant) is that the (electronic!) instantaneous breaker of the starter has an adjustment setting of "1" (i.e. a vertical line at 600amps on the current-time curve). My understanding is that being electronic it's not relying on magnetic forces of an "instantaneous" current.

Can I use this setting of "1" (600amps) to protect my 620amps worth of transformer secondary tap conductors? If set to "1", doesn't this provide the same protection as a electronic breaker with long time, short time, inverse time capabilities (i.e a breaker analogous to a thermal-magnetic breaker)? Is 240.21.(C)(2) fulfilled?​

You have included a lot of stuff but provided no information about the nameplate rating of the transformer such as the primary and specifically the secondary voltage and if you the has been an attempt to comply with the applicable section of NEC 450 regarding transformers.
Off hand I would guess that be simply by providing instantaneous protection doesn't cut it.
 

CobyRupert

Member
Location
NY
Let me clarify, the secondary of the 500kva transformer is 480V, I thought that was implied. The primary is 4160V. I don't believe protection of the transformer (Article 450) is an issue.
My inquiring is in regards to whether 620 amps of secondary tap conductors (<10') terminating on a 600amp electronic instantaneous (only) circuit breaker, where the instantaneous setting is set to 600 amps, complies with 240.21(C).

Or perhaps, the question is really: what is the nature of an electronic "instantaneous" only circuit breaker when set to "1" (e.g. a 600 amp breaker , when instantaneous trip is set to 600A)? (this is how 90% of the world thinks breakers trip anyways)

Doesn't it provide as much protection as thermal-magnetic breaker, or a breaker that has long-time, short-time, I^2t, settings? (i.e. don't long-time, short-time, I^2t, settings (thermal overload characteristics) become irrelevant at this instantaneous setting)?
 
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templdl

Senior Member
Location
Wisconsin
I will assume that it is a 480v 3ph3w delta where the transformer overload protection is provided by the PRI OCPD.
Then I would agree that the secondary instantaneous only breaker would be responding to a fault in the VFD.
Also, I agree that an instantaneous breaker would respond instantaneously rather than the TCC as a TM breaker would have.
But I would be looking at the response to a VFD issue as being the objective and not the transfortmer.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I don't see an instantaneous only device as being an overcurrent device based on the Article 100 definitions. If the transformer is not a delta/delta transformer with the primary OCPD sized to protect the secondary conductors, I see this installation as a violation.
 

CobyRupert

Member
Location
NY
I don't see an instantaneous only device as being an overcurrent device based on the Article 100 definitions. If the transformer is not a delta/delta transformer with the primary OCPD sized to protect the secondary conductors, I see this installation as a violation.

No, the secondary conductors are not protected by the primary OPCD.

Normally, if there was a thermal-magnetic breaker, I wouldn't question the transformer secondary conductors being protected by this downstream breaker in accordance with NEC 240.21(C). Conductors are protected because the downstream 600 amp breaker is protecting 620 amps of upstream cable (a rare case where this is allowed) from overload, and the limited length of the conductors (10' or 25') enclosed in raceway protect the conductors from short circuit.

So, maybe the question is, as you pointed out, does an electronic instantaneous breaker, set to "1" (600 amps) protect against overcurrent (in addition to short circuit/gnd fault); and does it qualify as an overcurrent protective device in compliance with 240.21(C)?

IMHO, it seems like it does qualify? It trips at 600amps instantaneously (& much quicker than a 600 fuse, or thermal-magnetic breaker experiencing an overload). Is my thinking wrong?

We don't normally think of instantaneous breakers as providing overload protection (only short circuit/gnd fault), but if they are set to "1" (i.e. to a current level less than the conductors they are protecting) does it matter that the time-current-curve is a vertical line?
 
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