Why do lamps blowing trip RCDs?

[NickD]

...cos if the lamp is in effect Class II with connections only to L and N (Edison screw; standard bayonet cap in unearthed pendant or batten), you would naively think there is no way the L and N can be in imbalance. But still the RCD trips. Definitely talking RCD trips here not MCB. I've seen mention of surge current spikes but never got to the bottom of it. Googling just seems to bring back forum postings that don't really answer the question. Cheers.

 

[MDJ]

...cos if the lamp is in effect Class II with connections only to L and N (Edison screw; standard bayonet cap in unearthed pendant or batten), you would naively think there is no way the L and N can be in imbalance. But still the RCD trips. Definitely talking RCD trips here not MCB. I've seen mention of surge current spikes but never got to the bottom of it. Googling just seems to bring back forum postings that don't really answer the question. Cheers.

Question for you, how does electricity flow through a circuit and make a meter run? think about that.

 

[NickD]

Question for you, how does electricity flow through a circuit and make a meter run? think about that.

In a purely resistive situation, in one end and out the other?

 

[MDJ]

A rcd trips on un-balance yes? so imagine a circuit which is energised then suddenely a surge occurs then an loss of load in milli seconds, the rcd would see un-balance and trip, a very common sinario, it happens very often and means the trip is probably working.

 

[Darkwood]

The rcd is a finely balanced unit relying on a very small imbalance between L and N ...when a filament lamp blows sometimes it leaves an ionised path through the gas atoms which act like a short circuit although it does collapse fast it can sometimes exceed the time/curve graph of the mcb.... as with any short circuit 1000's of amps may flow and the magnetic field associated with this is very large and can saturated the fine balancing fields of the rcd causing it to trip.

 

[MDJ]

The rcd is a finely balanced unit relying on a very small imbalance between L and N ...when a filament lamp blows sometimes it leaves an ionised path through the gas atoms which act like a short circuit although it does collapse fast it can sometimes exceed the time/curve graph of the mcb.... as with any short circuit 1000's of amps may flow and the magnetic field associated with this is very large and can saturated the fine balancing fields of the rcd causing it to trip.

Thats what I meant to say, but not quite as professionally put lol

 

[NickD]

The simple view of the situation would be
1. normal current flowing down L, through filament, back up N
(now filament starts to fail to a short(ish) L-N circuit)
2. failure current flowing down L, through filament, back up N
(now filament blows to open circuit)
3. no current flowing down L, through filament, or back up N

In each of those three the L and N are in balance and the RCD won't trip - so I guess there's something missing with that simple view, my question is what does it miss?

<Edit: posted this after darkwood's useful post which I thinks answers the question>

 

[NickD]

The rcd is a finely balanced unit relying on a very small imbalance between L and N ...when a filament lamp blows sometimes it leaves an ionised path through the gas atoms which act like a short circuit although it does collapse fast it can sometimes exceed the time/curve graph of the mcb.... as with any short circuit 1000's of amps may flow and the magnetic field associated with this is very large and can saturated the fine balancing fields of the rcd causing it to trip.

So it's like something horrible and inductive going on during the arc period, throwing the L & N into imbalance, am I on the right track?

 

[nickblake]

...cos if the lamp is in effect Class II with connections only to L and N (Edison screw; standard bayonet cap in unearthed pendant or batten), you would naively think there is no way the L and N can be in imbalance. But still the RCD trips. Definitely talking RCD trips here not MCB. I've seen mention of surge current spikes but never got to the bottom of it. Googling just seems to bring back forum postings that don't really answer the question. Cheers.

i think MDJ and Darkwood have answered the question

 

[Darkwood]

You get a massive magnetic field passing through L an N inside the RCD ..this can saturate the fine monitoring coils of the rcd and then its in the lap of the gods to whether it trips.

 

[NickD]

You get a massive magnetic field passing through L an N inside the RCD ..this can saturate the fine monitoring coils of the rcd and then its in the lap of the gods to whether it trips.

Oh right so not inductive effect so much as large fault current causing magnetic effects in/around the RCD coil and upscrewing it. Gotcha. Many thanks.

 

[uksparks]

The simple view of the situation would be
1. normal current flowing down L, through filament, back up N
(now filament starts to fail to a short(ish) L-N circuit)
2. failure current flowing down L, through filament, back up N
(now filament blows to open circuit)
3. no current flowing down L, through filament, or back up N

In each of those three the L and N are in balance and the RCD won't trip - so I guess there's something missing with that simple view, my question is what does it miss?

<Edit: posted this after darkwood's useful post which I thinks answers the question>

If you want to get technical about it and correct me if I'm wrong, current flows from positive / Line to negative / Neuteal and to complicate it the electrons flow from negative to positive, but you should know that.

 

[spark 68]

If you want to get technical about it and correct me if I'm wrong, current flows from positive / Line to negative / Neuteal and to complicate it the electrons flow from negative to positive, but you should know that.

Ah! conventional current flow you are talking about ? however this is AC so they are just bobbing to and fro 50 times a second. lol

 

[telectrix]

the answer to the OP is the same one as "why do dogs lick their bollox"...." because they can".

 

[uksparks]

True hence alternating current