How to calculate maximum switchable load

[wonderwire]

I am installing a PIR to turn on a fan and 5 recessed lights.

It says the maximum switchable load is as follows:

Are these limits meant to be added together, or individual restrictions for the circuit?

How much "slack" are in these figures usually?

My fan is 77W, and each lamp is 6.5W. Will this work? What would likely happen if this "failed"? Fire / Explosion / Darkness?

 

[EFLImpudence]

If the maximum it can switch is 75W for a fan then I don't suppose 77W will make much difference but it won't be suitable for the lights as well.

 

[wonderwire]

Thanks @EFLImpudence. So how do I read other PIR specs where they just say "Switching capacities 110V-130V max 500W, 220-240V max 1200W.". Would this manage to switch the lights and the fan?

Reference: http://www.toolstation.com/shop/Lig.../sd3296/360°+Ceiling+PIR+Single+Sensor/p65562

 

[EFLImpudence]

As the only rating given is half your first example, the other types would presumably be halved as well so unlikely.

There may be more details supplied with the item.



You could have it switch the lights and a relay for the fan.

 

[EFLImpudence]

If you used a fan with a timer, that would be alright as the trigger to switch it on uses very little current.

 

[wonderwire]

Thanks, you mean for example a fan with a delay that comes on 2 mins after the switch is triggered?

 

[ericmark]

500W fluorescent not suitable for discharge lighting but a fluorescent tube is discharge lighting. I would guess it is looking at the time it takes for high bay lights to fire up, you would not what them to switch off before they got going.

The problem is with the back EMF when switching magnetic equipment and since you don't really know what that is then it is always going to involve some guess work and the spec is at best a guide. Frequency is important where the work force arrives at 9 and leaves a 5 then in real terms it only switches twice a day. But in a corridor or toilet it may be triggered many times a day.

Having a PIR work a relay may be harder than simply fitting two PIR's and with a room like a toilet the idea of setting the delay time for fan longer than for the lights would make some sense.

500/110 = 4.something and 1200/240 = 4.something so it seems likely it has a 5A relay built in. PIR's with relays are easier to work with than those with solid state switching so I would select one with a relay.

 

[endecotp]

You should assume that all switching devices should be derated for inductive loads such as fans.

The derating from 2400W to 75W seems surprisingly large, to me. See if you can find others with detailed datasheets.

 

[EFLImpudence]

Thanks, you mean for example a fan with a delay that comes on 2 mins after the switch is triggered?

No, fans that run on after the light goes out.

You could set it to minimum - usually around two minutes.

 

[endecotp]

If using a different fan is an option, you may be able to get a lower power one.

 

[wonderwire]

What's the most likely result if I exceed the load on the PIR? Is it dangerous?

 

[EFLImpudence]

The PIR will burn out.

 

[wonderwire]

Please could you explain why the fan "downrates" 2400W to just 75W? What is happening here?

 

[endecotp]

I could try to explain, but it is probably more effective to google for "switching inductive loads"; you might find a good explanation with pictures.

Basically, an inductor (such as the winding of a motor) stores energy in the form of magnetic flux. When you remove the power, that energy goes somewhere. That might take the form of a spark or arc across the contacts of the switch in the relay in the PIR. That spark will eventually erode the contacts so that they don't work reliably. Or, it may cause the contacts to weld together.

The likely failure mode is localised melting / burning, but prpbably not an actual fire that escapes from the device. But only eventually - it will probably work fine for ages before failing.