Open Delta Fusing
Open Delta Fusing
(OP)
We have an open delta transformer configuration used for metering purposes. This is an old installation so only two legs of the delta are fused with the middle leg tied to ground and no fuse.
Is it still standard practice to fuse only the two outter legs and not the third (grounded) leg? I can't picture a scenario where the two fuses will not see the current (and therefore open the fuses). With motors, this set-up is not allowed any more (overloads). I was wondering if the same can be said for an open delta transformer configuration?
Is it still standard practice to fuse only the two outter legs and not the third (grounded) leg? I can't picture a scenario where the two fuses will not see the current (and therefore open the fuses). With motors, this set-up is not allowed any more (overloads). I was wondering if the same can be said for an open delta transformer configuration?






RE: Open Delta Fusing
RE: Open Delta Fusing
See NEC 430.36 and 37. (2002 Edition).
RE: Open Delta Fusing
Even if I am wrong about that then I can not see why the grounded leg would see an OC under any conditions (fault line-line will be picked up by other two fuses and fault to ground can not happen since the leg is already grounded).
Thanks for the posts and helping me to answer the question with reasoning.
RE: Open Delta Fusing
buzzp, are you asking about metering-VT primaries or secondaries? For 2-element 3ø 3-wire metering with two VTs, 3 fuses are typically used with an ungrounded open-delta primary, but the corresponding open-delta secondary often has the jumpered connection {bø} grounded, with a pair of fuses in the other two secondary leads—consistent with IEEE C57.13.3.
Another case with 3ø 4-wire 2½-element metering is an open-wye primary using two VTs/two primary fuses with their common connection grounded, with the same for the VT-pair open-wye secondary.
RE: Open Delta Fusing
You described our installation very well in your first paragraph. If I would have thought about it some more I would have understood why this third fuse is not needed. Thanks for the IEEE reference. This power stuff is new to me so you have to be patient :) Thanks again to all.
RE: Open Delta Fusing
The ‘four fuse’ practice is mentioned in GEP-9186-6 for current-limiting primary fuses in open-delta applications. General Electric’s rationale is that a current-limiting fuse should not simultaneously energize two VT primaries. [I have discovered too many MV fuses that seem to have opened from merely looking at them crosseyed, and no other apparent reason.]
I misstated the IEEE reference and should have said, “…[fusing of ungrounded-secondary leads] consistent with the secondary-grounding recommendations of IEEE C57.13.3.”
RE: Open Delta Fusing
RE: Open Delta Fusing
RE: Open Delta Fusing
Bung
Life is non-linear...
RE: Open Delta Fusing
I would think it would be but have found that our plants do not have a ground on the open leg.
RE: Open Delta Fusing
It is assumed that the subject CTs are for an application like 2-element metering of a 3-wire delta (or 3-wire wye) circuit.
ANSI/IEEE C57.13.3-1983 calls out secondary-side instrument-transformer grounding, but may be indirect where used in applications like transformer-differential relaying where, for delta-wye power transformers, delta-connected CTs are grounded via the interconnected-wye CTs at the oppsite boundary of the differrential zone. See C57.13.3 §2.
Electrostatically-induced potential may exceed the secondary-component insulation-withstand rating if left ungrounded.
RE: Open Delta Fusing
I will be requesting a ground be added to these CT circuits.
RE: Open Delta Fusing
Okay, we have 2 CT's metering a 3 phase Y circuit. Each phase has its own ammeter (no power meters are hooked to these). The B-phase is grounded. My question: is there a preference as to whether this grounding would be after the b-phase meter or before the b-phase meter? The IEEE likes to see them grounded at the first point of application. This would mean it would be grounded before the meter. However, all of our open delta CT metering circuits are set-up with the ground after the meter input. I can see a difference if we were measuring power flow but the meter does not care which side of its input is grounded. Anyone have any reasons why I should keep the ground intact after the meter? I appreciate the help.
RE: Open Delta Fusing
buzz, are you referring to the case where two CTs are configured to indicate all three phase currents? See page 37+ of GET-97D, at www.
RE: Open Delta Fusing
I found the drawing in the GE document on page 37 figure 39. This figure shows a ground at the CT locations (presumably since the R's in this figure are the wire lead resistances).
If you imagined, now, the R's are our current meters with the Z's being replaced with wire. Would you put the ground where it is on this figure or move the ground to the other side of "R2" ('B' phase meter)? Thank you.
RE: Open Delta Fusing
Either way will work to preserve secondary insulation, in indstrial switchgear it is most common to ground at the CT-common jumper via a “shorting block” similar to a Marathon 150xSC-series device.
RE: Open Delta Fusing
RE: Open Delta Fusing
RE: Open Delta Fusing
Busbar, I can apparently only give one star per thread per person so sorry I can not give you another. Thanks though.
RE: Open Delta Fusing
Very happy to help, buzz. This message board is a very fitting use of the internet.