Discrimination headache!!!

joebell25 · Jan 27, 2017

Thought I'd try out this ElectricalOM software for a new installation I'm designing. However come up with abit of an issue with the discrimination on 2 of the devices. Single phase sub feed from DB2 (60898 type c 63A 10kA) to DB3, Final circuit from DB3 (61009 30mA 32A type c 10kA RCBO)

Time v Current curve for the devices shows no discrimination for over current protection.

Any ideas or advice?

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jackhammerJIM · Jan 27, 2017

You won't get full discrimination between 2 mcbs . Feed from a switch fuse will solve your problem .

joebell25 · Jan 27, 2017

Trouble is I've got 15 of these DBs fed from DB2, that's a hell of a lot of switch fuses.

Leesparkykent · Jan 27, 2017

Using two circuit breakers in series the upstream device would have to be in the region of 125A to fully discriminate with the 32A.

joebell25 · Jan 27, 2017

Thanks for your help.

Leesparkykent · Jan 27, 2017

joebell25 said:
Trouble is I've got 15 of these DBs fed from DB2, that's a hell of a lot of switch fuses.

Sounds like a poor design. What about using something like a ryefield or Lucy board.

RY18 - 18 Way Ryefield Board taking 60A Fuse Units, 18 Way TPN Fuseboard - https://www.sparksdirect.co.uk/18-way-ryefield-board-taking-60a-fuse-units-18-way-tpn-fuseboard#.WIu7muvfWK1

MODECSOFT · Feb 3, 2017

Hi,

@joebell25 If you need any help with the software don't hesitate to contact us or open a support ticket

davesparks · Feb 3, 2017

jackhammerJIM said:
You won't get full discrimination between 2 mcbs . Feed from a switch fuse will solve your problem .

Just use a DB with fuses instead of mcbs, such as the MEM Excel series DBs

Chr!s · Feb 3, 2017

joebell25 said:
Thought I'd try out this ElectricalOM software for a new installation I'm designing. However come up with abit of an issue with the discrimination on 2 of the devices. Single phase sub feed from DB2 (60898 type c 63A 10kA) to DB3, Final circuit from DB3 (61009 30mA 32A type c 10kA RCBO)

Time v Current curve for the devices shows no discrimination for over current protection.

Any ideas or advice?

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View attachment 35355

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Hi, that's a lot of tiers, try reducing, also you need to ensure your fault current analysis is correct.

MODECSOFT · Feb 4, 2017

MODECSOFT said:
Hi,

@joebell25 If you need any help with the software don't hesitate to contact us or open a support ticket

I forgot to mention that in the ElectricalOM - Getting Started Tutorial we show an example on how to resolve discrimination errors: Protective Devices Discrimination Problems - https://electricalom.com/app/help/ProtectiveDevicesDiscriminationP.html

Flanders · Feb 5, 2017

You nead to use an mccb board or as Dave and Lee says fuse boards to the supply distribution circuits so you can get discrimination

Big Sausage · Feb 6, 2017

Red-Spot fuse board fitted with BS88's would normally sort it. Just be weary though as some combinations of fuses and MCBs still won't fully discriminate.