Logic problem in OSG - trivial

Ziggie over 3 years ago

Page 79 OSG (blue edition)

Lcn is max fault current .......although the breaker may no longer be usable.

Lcs is max fault current .......can interupt safely without loss of performance.

.........

For domestic installations, the PFC is unlikely to exceed 6kA , up to which value the Lcn will equal Lcs.

usable , maybe usable, certainly usable?

Parents

0 Chris Pearson over 3 years ago

There is no faulty logic in the OSG or indeed BS 60898.

When I_cs = I_cn, it means that there is no range where an MCB would safely break a current but not remain serviceable.

It's all about design.

If the PSSC is 5 kA, you may select a 6 kA MCB.

If it is 7 kA, you may select a 10 kA MCB because I_cs = 7.5 kA.

If it is 8 kA, you need at least a 16 kA MCB because where I_cn is > 10 kA, I_cs = 0.5 x I_cn. I suspect that when you get into this territory, there will be something before the MCB.
Cancel
Vote Up 0 Vote Down

Cancel

Reply

0 Chris Pearson over 3 years ago

There is no faulty logic in the OSG or indeed BS 60898.

When I_cs = I_cn, it means that there is no range where an MCB would safely break a current but not remain serviceable.

It's all about design.

If the PSSC is 5 kA, you may select a 6 kA MCB.

If it is 7 kA, you may select a 10 kA MCB because I_cs = 7.5 kA.

If it is 8 kA, you need at least a 16 kA MCB because where I_cn is > 10 kA, I_cs = 0.5 x I_cn. I suspect that when you get into this territory, there will be something before the MCB.
Cancel
Vote Up 0 Vote Down

Cancel

Children

No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Logic problem in OSG - trivial

Join us to get the best from IET EngX.

Community Rules & Guidelines