Maximum zs calculation

[bigspark17]

discrimination.

Between? The op hasn't stated what supplies the main board,

 

[uksparks]

discrimination.

I suppose, but surely that depends on what the largest mcb in the remote end is?

 

[Leesparkykent]

Between? The op hasn't stated what supplies the main board,

between MCB's...The OP stated that he tends so supply his distribution circuit with a 63A C type.

 

[Leesparkykent]

I suppose, but surely that depends on what the largest mcb in the remote end is?

A 6A mcb would only offer partial discrimination with the 63A.

 

[bigspark17]

Between which mcbs. Hes only stated 63amp on sub main, the main board could be 200A 3phase supply and new extension 6A and 16A radials! A type B would give you more room on your zs margins if compatible to whats on your load side.

 

[Leesparkykent]

Between which mcbs. Hes only stated 63amp on sub main, the main board could be 200A 3phase supply and new extension 6A and 16A radials! A type B would give you more room on your zs margins if compatible to whats on your load side.

Exactly my point......sub main has a 63A mcb protecting it, final circuits connected to sub board are also on MCB's.

 

[Risteard]

Why have you got a problem serving a sub board off an mcb this is very common.?!

It won't discriminate for a fault.

 

[davesparks]

Why have you got a problem serving a sub board off an mcb this is very common.?!

I know it's very common. Our workshop is fed by a 63A type C mcb, a colleague plugged a light in for testing into a circuit fed from a 16A type B rcbo and when the lamp strike relay closed it took out the 63A mcb before the 16 or the 13A plug fuse.

MCBs don't fully discriminate with MCBs, regardless of what rating they are. Just look at the time/current characteristics.

 

[davesparks]

I suppose, but surely that depends on what the largest mcb in the remote end is?

Nope, a 6A mcb won't fully discriminate with a 63A under fault conditions.

If this care home is anything like the ones I've worked in there could be all sorts of problems if a number of rooms all lose power at once.
I've come unstuck once when isolating a lighting circuit knocked out part of the nursecall system, I switched it back on within 30 seconds but it took a lot longer to silence all the alarms and to settle things with the care supervisor etc.

 

[malcolmsanford]

1- don't feed a submain from an mcb
2- go up a cable size or two to lower the Zs

And that is spot on. A fuse will give you far more flexibility with tripping characteristics,

MCB's are far too sensitive for a distribution circuit and you can never fully rule out a final circuit tripping out a main MCB. MCCB's are slightly better but are far to expensive.

Fuses are cheaper, giving you more flexibilty especially now in many non industrial situations where you have in rush currents to take into account on switch mode equipment, AC systems, HF Flourescent lighting and even LED drivers.

Protect your final circuits by MCb's by all means, far safer, but a distribution circuit should always have fuse protection as a first choice.