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Warning! 25A switch won't handle 18A for long.

Posted: Mon Apr 15, 2019 8:36 pm
by simmit
A warning to others who, like me (idiot), take electrical component ratings at face value.

I used a 25A rated switch to manually operate rad fan (18A current draw rating) but it didn't last long. Simply it melted the switch plastic body and could easily have resulted in a fire. Nasty!
Should have done a proper job using a relay.
Can anyone give me a reliable ampere rating for Scimitar rad fan relay (Middlebridge in particular)?

Warning! 25A switch won't handle 18A for long.

Posted: Mon Apr 15, 2019 9:02 pm
by peter freeman
The MB fan motor fuse value is not listed but I would think 40amp should be ok. being two speed the two coils are fused by 25 amp fuses but that has nothing to do with the fan current. 30/40 amp relays are readily available.

Warning! 25A switch won't handle 18A for long.

Posted: Mon Apr 15, 2019 9:21 pm
by philhoward
Did you use a switch with 25A written on it, or one rated at a genuine 25A DC? 25A is a lot for a switch to cope with - it would have to be very substantial indeed hence why anything over about 10A is generally relayed.

Warning! 25A switch won't handle 18A for long.

Posted: Mon Apr 15, 2019 9:46 pm
by scimitargtc
My old foreman swore by relays but if you must, then use double what the load is I’d agree a 40 amp switch for a 25a current load but that’s a huge switch far easier and safer to correctly use relays to handle the power

Warning! 25A switch won't handle 18A for long.

Posted: Mon Apr 15, 2019 10:38 pm
by simmit
philhoward wrote:
Mon Apr 15, 2019 9:21 pm
Did you use a switch with 25A written on it, or one rated at a genuine 25A DC? 25A is a lot for a switch to cope with - it would have to be very substantial indeed hence why anything over about 10A is generally relayed.
Used switch from Halfords which packaging clearly claimed it was 25A.

Warning! 25A switch won't handle 18A for long.

Posted: Mon Apr 15, 2019 10:48 pm
by Roger Pennington
Is your otter switch faulty? You shouldn't need to use a manual switch in the current weather? (of course a relay is always a good idea to protect both)

Warning! 25A switch won't handle 18A for long.

Posted: Mon Apr 15, 2019 11:42 pm
by simmit
Roger Pennington wrote:
Mon Apr 15, 2019 10:48 pm
Is your otter switch faulty? You shouldn't need to use a manual switch in the current weather? (of course a relay is always a good idea to protect both)
Modified cooling system. No Otter switch. Ally rad with single speed 16" Spal fan pulling about 18A. Intend to fit auto temperature switch but meantime it's manual.

Warning! 25A switch won't handle 18A for long.

Posted: Mon Apr 15, 2019 11:50 pm
by Roger Pennington
Well, in the end, of course it's your car, and your decision, but that sounds a bit like spoiling the ship for a ha'p'orth of tar? Hopefully you won't forget the switch...

Warning! 25A switch won't handle 18A for long.

Posted: Tue Apr 16, 2019 12:48 am
by giddo
A couple of points occure to me on reading the above
1) was the 25A rating on the switch AC or DC ? AC current is much easier to switch as it will fall to zero (usually) 100 times a second limiting the arcing time. Arcing causes burning of the contact faces which results in higher contact resistance and heating.
2) Usually the quoted current drawn by a DC motor is the full load current, ie when running at normal speed pushing air through. At the moment of switch on the current will be limited only by the resistance of the windings. As the motor speeds up the back EMF rises and the current falls. If you have a good quality resistance meter try measuring the resistance of the motor at standstill; if it were 0.5ohms then that would be 28 amps stall current ( remember you've probably got 14v from the alternator) Sadly I cant remember typical values but the above suggested 2:1 or even 4:1 is sensible.

Warning! 25A switch won't handle 18A for long.

Posted: Tue Apr 16, 2019 6:38 am
by philhoward
simmit wrote:
Mon Apr 15, 2019 10:38 pm
philhoward wrote:
Mon Apr 15, 2019 9:21 pm
Did you use a switch with 25A written on it, or one rated at a genuine 25A DC? 25A is a lot for a switch to cope with - it would have to be very substantial indeed hence why anything over about 10A is generally relayed.
Used switch from Halfords which packaging clearly claimed it was 25A.
My Engineers brain sees that switch and doubts it can switch a 16” fan - will pop into a Halfords for a closer look.

As giddo says, a fan isn’t a constant load despite a constant current being quoted.

I would also suggest looking into an automatic temperature switch and a relay for the fan sooner rather than later (40A relay by the way).

Warning! 25A switch won't handle 18A for long.

Posted: Tue Apr 16, 2019 6:17 pm
by simmit
Thanks guys. Interesting discussion.

Giddo.
My cheapo meter gives 0.3 to 0.4 Ohms motor resistance at rest.

Phil.
It was always the intention to have temperature switch and relay but at the time there was not time to do so.
I readily accept, Phil, that a bigger relay is likely to be less stressed than a smaller one but as Middlebridge fuses (which I intend to use) for fan motor circuits are 25A the relays will never see more (will they?), so is 40A a tad generous?

Forgive this electrical numpty for asking daft question but just wanting to know more about the subject.

Warning! 25A switch won't handle 18A for long.

Posted: Tue Apr 16, 2019 6:24 pm
by peter freeman
Like I said here is no indication of what the rad fan fuse value should be - only the two relays.

Warning! 25A switch won't handle 18A for long.

Posted: Tue Apr 16, 2019 7:08 pm
by simmit
peter freeman wrote:
Tue Apr 16, 2019 6:24 pm
Like I said here is no indication of what the rad fan fuse value should be - only the two relays.
Thanks Peter.
I traced wiring to F17 (cockpit, nearside, front bulkhead) and found 25A fuse, functional but showing signs of distress. Of course, I've no idea if 25A is original spec'n or if someone over the years has put in a wrong 'un.
As an aside, a 25A fuse to the fan slow speed relay is surely overkill as it's only powering the coil. The 25A fuse to the high speed relay does also supply indicators but even so, 25A may be a bit over the top.
I'm not an electrician and struggle with the logic for some of the Middlebridge circuitry. Did they take a Ford loom, fuse boxes, relays and simply adapt to fit?

Warning! 25A switch won't handle 18A for long.

Posted: Tue Apr 16, 2019 7:35 pm
by peter freeman
Car fuses are often over rated he MGF EPAS system has two fuses 10A and 40A I fit 7.5A and 30A and think that the 7.5 is more than needed anyway - 5 is probably nearer the mark. 25 seems a bit low unless its a slow blow fuse.