Amd2 SPD on smoke alarms

[Electro-tech]

I think it was mentioned somewhere on another thread that Aico alarms have surge protection built in.

 

[pc1966]

I think it was mentioned somewhere on another thread that Aico alarms have surge protection built in.

All competent mains powered electronics has some degree of surge protection simply to meet the minimum EMC regulations for immunity. However, it is usually only for small surges (typically 1.5kV is specified) and from not-very-low impedance sources (ie. the CAT-III sort of location at end of final circuit).

Having a SPD at the DB should keep any surges down-stream to under that sort of value, and so they can then deal with the short over-voltage OK.

As for the "10m distance" aspect it is more complicated. If you are trying to coordinate SPD (say Type 1 at input, Type 2 and DB further away) then you want the first SPD (high power, higher flash-over voltage) to conduct first, and only what gets past is then clamped by the second lower-power / lower-voltage SPD. To achieve that selectivity you need some impedance, and for that read inductance, so usually a minimum of 10m of cable, or a dedicated air-cored coil (rare). Inductors often have ferrite or iron cores to increase inductance for a given size, but under big surge conditions those cores saturate and the inductance plunges to the air-cored equivalent, so you might as well go for air-cored to begin with!

It is also highly dependent on what you are trying to protect, and where the lighting might hit. In most cases it is a far away strike coming in via power cables and the first SPD it finds does the job. If very near than all sorts of cables within the installation and/or to extraneous parts can pick up the induced current and cause grief. Usually I would only see the point in a SPD at a sub-board if very far away or other reasons to expect high local current (LPS present, big motors likely to dump switch off spikes, etc, etc).

 

[pc1966]

Just to say I have used the Kingsmill stuff and they are good, if not cheap. But you also have to balance the realistic needs for protection in a given type of installation against what they are trying to sell!

 

[Lister1987]

Just to say I have used the Kingsmill stuff and they are good, if not cheap.

Surely they're (bread) crumby?