Transformer and MCCB Questions

[QuestionableSpark]

Hello All.
New here and will be flooding with terrible questions for my own knowledge. Just trying to learn so some questions like this maybe very simple and I'm just daft

1) On site we have a Aux Transformer,
From the LV side of the transformer we have an MCCB Board with a few circuits.
Now on the Bs7671 test sheets, it asks at the top of DB schedule what the OCPD is for this board.
Obviously it is fed directly from the transformer so I assume this would be put down as N/A since there isn't an OCPD?


2) On the MCCB itself it has the ratings etc, again on the BS7671 test cert we put the OCPD for the circuit being the 60947-2 MCCB however how do I know what Type?
tmax xt3n 250 is the MCCB

Just to clarify, I don't fill in test sheets or anythin I just give lads a hand ATM. But trying to work out how to fill these in.

 

[Julie.]

Normally there should be a mccb on the incomer to the board from the transformer, then onto the bus, then the outgoing circuits (often mccbs as well)

So the ocpd for the board would be the mccb on the incomer, and each circuit would have their own ocpd in the form of a mcb or mccb.

Mccbs don't generally have types, the type refers to the "instantaneous" portion of the curve.

So all mcb have the same curve (trips at 160s@2×, 20s@4×, 5s@10× etc) , but different points at which the "instantaneous" kicks in B=@5x, C=@10x ... and so on

Mccbs have selectable curves both in terms of the curve and the "instantaneous " - which could be time delayed!

So you could have the 250A incoming mccb set to 250A, with 40secs at 4× and an instantaneous set at 30× with a 0.5 sec delay.
AND
The same 250A mccb on one of the outgoing circuits set to 150A, with 20secs at 4×, and the instantaneous at 10× with no delay.

So it would be a n/a for the type on the form, but you would have to enter both the mccb rating AND the settings to make sense.

If you just have the rating of the mccb it would be useless as you could have 315A mccb supplied from 250A, which is valid because of suitable settings. But looks wrong!

 

[QuestionableSpark]

Normally there should be a mccb on the incomer to the board from the transformer, then onto the bus, then the outgoing circuits (often mccbs as well)

So the ocpd for the board would be the mccb on the incomer, and each circuit would have their own ocpd in the form of a mcb or mccb.

Mccbs don't generally have types, the type refers to the "instantaneous" portion of the curve.

So all mcb have the same curve (trips at 160s@2×, 20s@4×, 5s@10× etc) , but different points at which the "instantaneous" kicks in B=@5x, C=@10x ... and so on

Mccbs have selectable curves both in terms of the curve and the "instantaneous " - which could be time delayed!

So you could have the 250A incoming mccb set to 250A, with 40secs at 4× and an instantaneous set at 30× with a 0.5 sec delay.
AND
The same 250A mccb on one of the outgoing circuits set to 150A, with 20secs at 4×, and the instantaneous at 10× with no delay.

So it would be a n/a for the type on the form, but you would have to enter both the mccb rating AND the settings to make sense.

If you just have the rating of the mccb it would be useless as you could have 315A mccb supplied from 250A, which is valid because of suitable settings. But looks wrong!

Hi Julie!
Thankyou for the answer.
This transformer has the HV side coming in and the LV Side is a cabinet with multiple MCCB installed. I'd upload a photo but apparently the file is too large!

As for the test form I see you point!
Now carrying on from this how do I find out the MAX ZS Permitted? As this is also on the test sheet, I can Google most things and find out but with MCCBs I don't seem to find anything

 

[Julie.]

OK, in that case there is no incomer, and the ocpd is actually the primary side protection, usually a fuse.

You have to calculate the max Zs based upon settings.

So you generally need to have a fault current in excess of the "instantaneous" setting. (Not always the case for a feeder rather than final circuit.)

If that is say 2185A then you need to take 95% of the voltage (230V) and devide by the current

So 0.95 x 230V = 218.5V
R = V/I = 218.5/2185 = 0.1 ohm

This would be when the cables are hot, so since we measure the Zs when cold we must take 80% of the value above, so in this case

Max Zs = 0.1 x 0.8 = 0.08 ohm.

Of course this needs to be done for each final circuit.

On a typical 250A circuit you may set it at 5x so 1250A - repeat the above calculation and you should have max Zs around 0.14 ohm.

 

[QuestionableSpark]

OK, in that case there is no incomer, and the ocpd is actually the primary side protection, usually a fuse.

You have to calculate the max Zs based upon settings.

So you generally need to have a fault current in excess of the "instantaneous" setting. (Not always the case for a feeder rather than final circuit.)

If that is say 2185A then you need to take 95% of the voltage (230V) and devide by the current

So 0.95 x 230V = 218.5V
R = V/I = 218.5/2185 = 0.1 ohm

This would be when the cables are hot, so since we measure the Zs when cold we must take 80% of the value above, so in this case

Max Zs = 0.1 x 0.8 = 0.08 ohm.

Of course this needs to be done for each final circuit.

On a typical 250A circuit you may set it at 5x so 1250A - repeat the above calculation and you should have max Zs around 0.14 ohm.

Fantastic, Thankyou Julie. Would this Calculation work for Max ZS of all breakers etc?
Also why do we need the 80% Value, when filling in a test sheet I though 100% ZS Value was to be used?

 

[pc1966]

Also why do we need the 80% Value, when filling in a test sheet I though 100% ZS Value was to be used?

It depends on your assumptions and point of testing.

The general use of 80% de-rating comes down to the assumption that you do the test cold, so conductors are lowest usual resistance, but you are designing to run them at 70C as max typical operating temp for PVC cables, at which point their resistance is about 20% higher.

So if you measure just after shut-down so it is "hot" as far as the system is concerned, then you don't need it.

Equally if your cables are quite over-sized due to VD instead of CCC being the limiting factor, or for any other reason the CCC is well above the OCPD rating, they won't get to 70C so you can use a smaller de-rating value.

 

[Julie.]

Fantastic, Thankyou Julie. Would this Calculation work for Max ZS of all breakers etc?
Also why do we need the 80% Value, when filling in a test sheet I though 100% ZS Value was to be used?

It depends where you get the max Zs value from.

In the standard, it is the hot value, (eg a 32A type B will have a Zs max of 1.37 ohm in the bs) so you would need to take 80% in order to estimate the resistance when cold - as you usually measure when cold. (80% x 1.37 = 1.1 ohm)

In the osg it is already at the 80% value - 1.1 ohm for 32A type B.