Breaker Coordination

[charzree]

The trip ratings of upstream and downstream circuit breakers should be same as I was told by a senior. Why cant the downstream circuit breaker be rated less than the upstream breakers. I'm not talking about the short circuit ratings, but the Ampere trip rating.

 

[Darkwood]

I haven't got a clue what this senior figure is on about, the fact you have to try to achieve short circuit discrimination against upstream devices automatically means that the upstream device will be rated much higher, it sounds like they haven't a clue what they are talking about.

If you can achieve full discrimination then a supply to a standard mcb board would usually require a 160amp rating to achieve this, running the same size ratings whether the trip curve is different or not will not achieve this in any way.

 

[Darkwood]

I will just add a note here as I just realised you are from Saudi Arabia...

Please note that the forum is based around the BS 7671 and other British Standards so our replies may not reflect local regulations if you are not UK based.
Having said this, discrimination works the same regardless of borders, how each country regulates to account for discrimination and what is required is up to them though, if you are confused or concerned by the information given to you then you should seek relevant information/regulations for your country.