Socket Breaker

[Gardner]

What should the magnetic trip value be for a 20amp radial general use circuit?

 

[Geordie Spark]

What should the magnetic trip value be for a 20amp radial general use circuit?

Here in the UK or there in the USA???

 

[Gardner]

Here in the UK or there in the USA???

Under BS7671 wiring regs.

 

[Deleted member 26818]

That would be instantaneous, approximately 10ms.

 

[Gardner]

That would be instantaneous, approximately 10ms.

Correct, my mistake, wordings a bit off. :icon12:

 

[Deleted member 26818]

An MCB would be expected to trip instantaneously within one half cycle.
In the UK, our supply has 50 cycles per second (50 Hertz).

 

[Gardner]

An MCB would be expected to trip instantaneously within one half cycle.
In the UK, our supply has 50 cycles per second (50 Hertz).

Is there a min or max instantaneous trip?

I would imagine that the earth fault loop impedance must be low enough for a none RCD breaker to clear in 0.5/50 of a cycle? Does the same disconnect time also apply to a L+N fault or is it the same for L+PE?

 

[Deleted member 26818]

It will all depend where on the cycle the fault occurs.
Maximum voltage and current is achieved at each peak.
Shouldn't matter whether the fault is L-E, or L-N.
Current values for instantaneous tripping, can be found in the tables in BS7671.
They are denoted by the straight lines before the line starts to curve in the tables.

 

[Gardner]

Do you have a page number?

 

[davesparks]

Is there a min or max instantaneous trip?

I would imagine that the earth fault loop impedance must be low enough for a none RCD breaker to clear in 0.5/50 of a cycle? Does the same disconnect time also apply to a L+N fault or is it the same for L+PE?

Yes, that is the whole point of carrying out Earth fault loop impedance tests.

The same does apply for a L - N fault, the mcb cannot detect what the nature of the fault is, just the magnitude of the current.

 

[Gardner]

Id imagine a L-N fault would be less of a hazard to humans in terms of shock, but it sounds like the regs want identical tripping for both L+PE and L+N.


So I guess I am lead to strike a balance between a breaker that has a high enough In trip not to blow on something like a vacuum cleaner starting up, but a low enough In to trip in 0.5/50ths of a cycle assuming a fault on the last socket on the radial since this will have the highest earth fault reading?

 

[davesparks]

Id imagine a L-N fault would be less of a hazard to humans in terms of shock, but it sounds like the regs want identical tripping for both L+PE and L+N.


So I guess I am lead to strike a balance between a breaker that has a high enough In trip not to blow on something like a vacuum cleaner starting up, but a low enough In to trip in 0.5/50ths of a cycle assuming a fault on the last socket on the radial since this will have the highest earth fault reading?

What is this question leading up to?

 

[Gardner]

What is this question leading up to?

The correct selection of instantaneous trip level for a breaker. The ones that can be ordered come in different trip curves, each with different instantaneous pick up levels. Some are only a few times the handle rating while others go to 12x or more.

If I am looking at it the wrong way let me know

 

[spark 68]

The correct selection of instantaneous trip level for a breaker. The ones that can be ordered come in different trip curves, each with different instantaneous pick up levels. Some are only a few times the handle rating while others go to 12x or more.

If I am looking at it the wrong way let me know

For general use sockets Type B is normally used, the instantaneous trip part is 5X In, these will be adequate for normal loads unless you have something specific in mind that may need a breaker requiring a higher threshold.

 

[Deleted member 26818]

MCBs whith higher instantaneous trip current values are used, where high start up currents are expected.
For example where there are DOL motors, lots of fluorescent lights, electric heaters, etc.