Blowing Fuse

[runnach]

I was always taught the simple rule was watts / volts = amps ......so eg 60w/12v =5 amps so you then select the fuse next size up so in my example 7.5 amps .

In practice people seem to fit what’s at hand with no thought given to good practise 13 amps typically fitted to any domestic appliance I have seen that often

In this case and with my understanding a 20. Amp would be the correct fuse,

 

[shortcircuit]

That’s good, the “why” was determined.

 

[harrow]

But a quality 15 amp blade fuse is designed to allow 15 amps continuously and will fail at a load of 20 to 30 amps.

 

[winks]

But a quality 15 amp blade fuse is designed to allow 15 amps continuously and will fail at a load of 20 to 30 amps.

It's all coming back now from electrical principals in 1969 City and Guilds. Coarse protection vs. Close protection. Thought I'd put all that behind me. Another vote for 20A though and keep a close eye on the fridge wiring.

Cheers

H

 

[Fazerloz]

Do not forget though that fuse is carrying other loads as well, not just the fridge.

 

[Fisherman]

Do not forget though that fuse is carrying other loads as well, not just the fridge.

I wondered about that to, but when you think about it the Awning light, and interior door lights will be leds and using little current. Also the fridge only draws 170a when moving, and these lights would be off then.

 

[shortcircuit]

Do not forget though that fuse is carrying other loads as well, not just the fridge.

The fuse is there to protect the cable and not to cover the total current of the various appliance attached, which logically would be under its rating

 

[Fazerloz]

I wondered about that to, but when you think about it the Awning light, and interior door lights will be leds and using little current. Also the fridge only draws 170a when moving, and these lights would be off then.

True but the lights are a unknown figure and what if he were to run his engine when parked up to charge batteries wouldn't the fridge automatically go onto 12v. I also thought fuse ratings should be 20% above actual load for continuous use. I could well be wrong but that's how i've always worked it.

 

[Fisherman]

The fuse is there to protect the cable and not to cover the total current of the various appliance attached, which logically would be under its rating

Which is why I reckon a 25a fuse would be ok if required.

Lets not forget that the fridge has its own internal fuses a 20a fuse for the fridge, and a 2a fuse for the fridge lighting.

 

[shortcircuit]

Which is why I reckon a 25a fuse would be ok if required.

Lets not forget that the fridge has its own internal fuses a 20a fuse for the fridge, and a 2a fuse for the fridge lighting.

And the size of the cable is capable of taking a load of 25A??????????????

 

[Fisherman]

And the size of the cable is capable of taking a load of 25A??????????????

yes it’s connected to a 25a slot.

 

[maingate]

yes it’s connected to a 25a slot.

The EBL leaves the factory marked up with fuse recommendations. The motorhome manufacturer does what it likes with the wiring fitted to the EBL. Manufacturers are noted for undersizing wiring on motorhomes .... that is why it is best to disregard electrical norms when talking about motorhomes (unless you have a selfbuild).

 

[Markd]

I'm afraid we may all be barking up the wrong tree.
The OP tells us that the circuit is permanently live - so it won't be supplying the fridge element.
From the description it is probably the supply for fridge light.
None of the loads should get anywhere near 15A.
The OP tells us that a larger fuse blew first and originally only 15A available.
It may be worth trying a higher fuse but I suspect that the issue will continue.

 

[Nabsim]

@Markd the OP may have meant that it isn’t turned off when you turn 12voff at the control panel. I believe the wording is similar for my EBL but may say permanently available. It is a circuit to provide fridge 12v, one internal light, electric step and I think maybe something else on mine.
With my setup you turn 12v off at control panel when you are going to move then turn it back on when you arrive. Never looked closely at circuit diagrams to see why but it may be to disable some battery equalisation circuit or switch mains availability on.
Needs the OP to say though

 

[r4dent]

[QUOTE="Fisherman, post: 1050321, member: 62221 "... the fridge only draws 170a when moving..."[/QUOTE]

I think this is a typo !

 

[in h]

But a quality 15 amp blade fuse is designed to allow 15 amps continuously and will fail at a load of 20 to 30 amps.

Not really. You should rate the fuse 30% above the normal current. Fuses are to protect cables from overheating. If the cable doesn't have a 30% overhead, fit a thicker cable.