Weighing up discrmination with the likelyhood of a fault

[DrDrreeeaaa]

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.

 

[raptor6458]

Being close to the submain setting doesnt really matter to the fault currents.
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.

 

[ScottyUK]

Are you using a dumb thermal-mag tripping unit or an adjustable electronic type? You will almost certainly find it easier to achieve discrimination using the electronic type. Having said that, the 160A breaker was a pretty good fit to the rule-of-thumb 1.6:1 ratio between cascaded devices.

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!

 

[DrDrreeeaaa]

All CBs are electronic. I have thoroughly investigated the manufacturer's data and concluded that it is not possible to discriminate between a set 200A downstream and a set 270 upstream.

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.

 

[DrDrreeeaaa]

Yeah, sorry guys, in the sketch the main switch should be set at 270A

 

[ScottyUK]

Symbols... yeah, just checking. Made sense for it to be that way.

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!