Discrimination

[securespark]

I am having a minor disagreement with my colleagues.

A job we have investigated has outside lights plugged into a ring via a 13A plug top.

There is random tripping of the mcb (no rcd in circuit).

I am suggesting that there must be discrimination between the 13A plugtop fuse and the B32 breaker, but my colleagues think it may be possible for the fault to trip the breaker first, leaving the cartridge fuse intact.

What do you guys think?

 

[kendor]

I'd treat the plugtop fuse as not part of the installation but as part of the appliance.
therefore the final protection to the final sub circuit is the MCB.

The discrimination rules would not take the plugtop fuse into the equation.

That's not to say that the plugtop fuse rating shouldn't be correct as of course it should be so and rated correctly for the application.

 

[Lectrician]

A plug top fuse has poor characteristics compared to other types. It is common for a B32 or less to trip before a 13amp plug top fuse. Especially if the ring is also loaded with other items.

 

[kendor]

Indeed, a plug top fuse falls under the same classification as a rewireable fuse.

 

[1974stephen]

Is the plug top fuse 3A, 5A or 13A?

 

[Adam_151]

They don't discriminate properly, 13A plugtop fuse and 32A Type 2 Breaker both seem to clear the fault simultaniously.

(that'll teach me to drop the kettle in the sink, wipe it dry and attempt to use it )

 

[kendor]

Is the plug top fuse 3A, 5A or 13A?

Although i see what you are getting at, the rating should be adequate for the purpose it's intended for therefore if the circuit it is protecting has a load current that calls for a 13A fuse then so be it (wiring from plug top should be rated for the job).

Therefore there may not be a possibility of derating the plugtop fuse yet the same fault would occur, it's the question of discrimination that's being asked and the answer is that the plugtop fuse is not considered part of the installation according to the regs and doesn't have to comply with the discrimination rules (as they stand today).

If the lights were hard wired and a fuse protected the final part of the circuit then this WOULD have to take discrimination into consideration.

 

[ban-all-sheds]

 

[kendor]

Perhaps you should point out the graph is for disconnection times and not related to the discrimination regs , in order to stop any confusion.

 

[andy]

I am having a minor disagreement with my colleagues.

A job we have investigated has outside lights plugged into a ring via a 13A plug top.

There is random tripping of the mcb (no rcd in circuit).

I am suggesting that there must be discrimination between the 13A plugtop fuse and the B32 breaker, but my colleagues think it may be possible for the fault to trip the breaker first, leaving the cartridge fuse intact.

What do you guys think?

i once had a direct short on a light i was testing (didnt realise live and neutral had touched). it tripped a B32 (not much else in use at the time) and the 5A in the plug was still OK.

 

[kai]

How then, would discrimination be achieved when a fixed appliance (room heater say) , wired through a 13amp FCU off the Ring be arranged?

 

[Adam_151]

Not easily, best solution is not to put fixed loads on the ring, especially not if they are heavy loads

 

[kendor]

How then, would discrimination be achieved when a fixed appliance (room heater say) , wired through a 13amp FCU off the Ring be arranged?

its a problem that has never been addressed, you don't have a lot of choice with a fused spur to using anything but a BS1362 in it. A solution maybe would be heaters( i presume you mean bathroom as they are the only fixed type i can think of that would spur off a ring circuit) on their own circuit?

 

[ban-all-sheds]

its a problem that has never been addressed, you don't have a lot of choice with a fused spur to using anything but a BS1362 in it. A solution maybe would be heaters( i presume you mean bathroom as they are the only fixed type i can think of that would spur off a ring circuit) on their own circuit?

That's the solution. It's not never been addressed - it's always been there, if anybody had ever been concerned enough about discrimination to implement it - either individual circuits, or a big fat cable on a big fat MCB so that the 1362 goes first.

 

[plugwash]

yep the MCB discriminating with plug fuse issue is something thats never really been thought through and applies just as much to portable stuff as fixed stuff.

the problem is that once you hit the fast trip point modern mcbs trip insanely fast (0.1 sec is iirc the requirement in the standard but if you look at wylexes info in thier catalogue they show tripping in less than a cycle!)