Weighing up discrmination with the likelyhood of a fault
Weighing up discrmination with the likelyhood of a fault
(OP)
Hello all,
I am involved in a LV system design I have sketched up in the attachment. Please open.
Discrimination at all levels is a requirement, but is difficult to achieve because the 270 main switch rating is so close to the 200 submain settings.
My design had a 2-off 160 CBs to each submain which discriminated with the 270 upstream.
The final design is shown in the sketch attached. Discrimination is achieved only for the subcircuits in each MDB. The engineer responsible rationalised that a fault upstream of these circuits is sufficiently unlikely that discrmination is not required.
What do you think of this compromise? opinions?
Michael.
I am involved in a LV system design I have sketched up in the attachment. Please open.
Discrimination at all levels is a requirement, but is difficult to achieve because the 270 main switch rating is so close to the 200 submain settings.
My design had a 2-off 160 CBs to each submain which discriminated with the 270 upstream.
The final design is shown in the sketch attached. Discrimination is achieved only for the subcircuits in each MDB. The engineer responsible rationalised that a fault upstream of these circuits is sufficiently unlikely that discrmination is not required.
What do you think of this compromise? opinions?
Michael.






RE: Weighing up discrmination with the likelyhood of a fault
And you'll never know when and where the fault will occur. Besides as what you have said discrimination is required at all levels. Dont go for an irrational explanation.
Consult your CB manufacturer for various setting of electronic type CBs.
RE: Weighing up discrmination with the likelyhood of a fault
Assuming your local code permits this, replace the 200A breakers with 160A and use a switch rather than a thermal-mag breaker as the incoming device at MDB-1 and MDB-2. That appears to meet the discimination requirements, but it depends what the load and diversity conditions at the boards are - are you saying that a 160A breaker is inadequate for the downstream loads?. If you have heavily laden 100A circuits and little diversity the source itself looks marginally sized.
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If we learn from our mistakes I'm getting a great education!
RE: Weighing up discrmination with the likelyhood of a fault
You think the explanation was irrational? He has basically argued a fault is unlikely on the busbars and on an underground cable, hence discrimination is not required upstream of these locations. I personally do not like it, however, I am young and not experienced at rationalising the requirements like this.
Scotty, The incoming device at the MDBs is a switch currently. We may be getting confused over symbols. If it is drawn with a circle on the line side it is a switch, a cross it is a CB.
RE: your second post, yes, one 160A breaker would not be enough. My proposal was two 160A breakers to each building, however, to avoid the obvious confusion and maintenane dangers, one would feed one load only (air compressor) while the second would feed distributed load throughout the building.
Yeah, you are right about the overall TF size. The system is operated such that only one building is energised at a time. A changeover arrangement was rejected by the client.
RE: Weighing up discrmination with the likelyhood of a fault
RE: Weighing up discrmination with the likelyhood of a fault
Sounds like you need a review meeting with your client. Present the options.
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If we learn from our mistakes I'm getting a great education!