When to use Fuse or Auto Reset breaker

Submitted: Wednesday, May 25, 2011 at 09:53

ThreadID: 86534 Views:13157 Replies:6 FollowUps:11

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KenInPerth

Hi all

Pondering issues as I do, I was thinking about "when should you use a fuse and when should you use an auto reset (thermal) breaker".

I am assuming there are some "best practices" that apply to the use of one over the other.

My assumptions are:

1. if the circuit had the chance of developing a hard fault (say something cuts into a cable) then you would want a fuse to blow and that would be the end of it - especially if this is a take off direct from the battey.

2. if the circuit is a high current circuit that could have transient overload (say a winch jamming) then the auto reset breaker would be the choice there??

As always I look forward to your responses.

Ken

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Reply By: Roughasguts - Wednesday, May 25, 2011 at 10:07

Wednesday, May 25, 2011 at 10:07

Use a Auto reset breaker on your trailer electric brakes ETC.

Cheers.

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Follow Up By: Member Al (Sunshine Coast) - Thursday, May 26, 2011 at 09:17

Thursday, May 26, 2011 at 09:17

Why?

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Allan

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Follow Up By: snoopyone - Thursday, May 26, 2011 at 13:14

Thursday, May 26, 2011 at 13:14

Probably becaues that is what is recommended by the makers of brake controller units.

If they overload and set it off it will reset and you will still have brakes whereas a fuse when blown will render brakes inoperative.

Probably you will have other reasons why to or not to but thats as I understand it.

Cheers

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Follow Up By: Member Al (Sunshine Coast) - Thursday, May 26, 2011 at 15:53

Thursday, May 26, 2011 at 15:53

OK Snoopy, I'll be the mug.
What causes them to overload?

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Allan

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Follow Up By: snoopyone - Thursday, May 26, 2011 at 15:55

Thursday, May 26, 2011 at 15:55

I have no idea .

merely repeating what I have read as to why you should use them

thought you would have more of an idea than me.

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Follow Up By: Member Al (Sunshine Coast) - Thursday, May 26, 2011 at 16:07

Thursday, May 26, 2011 at 16:07

No mate, I have never had anything to do with electric brakes or their controllers.
I know how they work and that's about it.
However, like any other electrical device, if the over-current protection kicks in then somethings crook and it could be a good idea to find out what it is than allow the power to continue in an on-off alternating fashion.
Seems to me that if the electric brakes are considered so important it may be advisable to have a monitor/alarm system to alert if the power is lost.

I'll do some searching on it.
Maybe someone else has been there and has some thoughts on this?

Cheers
Allan

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Follow Up By: snoopyone - Thursday, May 26, 2011 at 16:27

Thursday, May 26, 2011 at 16:27

The Prodigy does have that as it has a little screen with digits that signify settings and conditions.
One of which is a small o.L which the instructions say is an overload situation

However they dont say what might cause one.

Could be a short or something but there was a thread on a forum recently that said they had got that signal and didnt know what caused it.

Does have another error code for a short when in idle mode

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Follow Up By: Roughasguts - Monday, May 30, 2011 at 06:55

Monday, May 30, 2011 at 06:55

Just a thought but any number of unrelated wires EG: using your cigarette lighter can cause a temporary short! (you only need one burn't earth wire in your loome and and may render your Elecrtic brakes useless. So as a back up and a pretty important one the fitting instructions mention install a 20 Amp re-settable Fuse.

Cheers

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Reply By: AlbyNSW - Wednesday, May 25, 2011 at 20:48

Reply By: Member - nick b - Wednesday, May 25, 2011 at 23:25

Reply By: Member Al (Sunshine Coast) - Thursday, May 26, 2011 at 11:29

Thursday, May 26, 2011 at 11:29

Hi Ken,

The reason for any protection, fuse or circuit breaker, is to disconnect a circuit where the current is exceeding a safe value and so avoid further damage and possibly fire. The application of an automatically resettable circuit breaker is normally restricted (in industry and domestic applications) to incorporation within appliances where an overload, as distinct from a fault, may occur. This may for example be a portable tool or battery charger. It is not a good idea to use auto-resettable protection with unsupervised loads as the repeated re-application of the supply may cause further damage than the original fault.

I would consider it best to be made aware of a fault by a blown fuse or opened manually-resettable circuit breaker than to have an auto-resettable breaker continue to repeatably apply power until the battery was flat or something worse occurs.

My preference is for manually resettable breakers where the circuit is rated above 10 Amps and appropriate fuses where the circuit is rated below 10 Amps.

I am not at all sure about protection for winches where the current is up to several hundred Amps. A circuit breaker capable of this would be very expensive and of questionable protective value. Some winches may incorporate protection by means of internal thermal sensing which controls the contactor (solenoid) but otherwise it is left to the operator to judge if the motor is reaching its temperature limit. A fusible link (really just a high-rated fuse) could be used in the winch supply cable to protect against a short-circuit but may well not discriminate between maximum winch current and a short-circuit. I would prefer a manual isolator installed at the battery to be closed only when actually using the winch. This has distinct advantages against catastrophic short circuit and possible fire in the event of a front-end collision.

Cheers
Allan

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Follow Up By: KenInPerth - Thursday, May 26, 2011 at 11:57

Thursday, May 26, 2011 at 11:57

Great info and explanation.

As you say, and when I was thinking about using auto reset breakers, I started to come to the conclusion that they would not be very good if there is a hard fault "downstream" and it kept re-applying the power. Worst case would come to a fire somewhere in the vehicle.

The Winch was probably a bad example from me but I will heed your points about isolation of such high current stuff.

Ken

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Follow Up By: Member Al (Sunshine Coast) - Thursday, May 26, 2011 at 13:01

Thursday, May 26, 2011 at 13:01

No Ken, I think your raising the issue of winch protection was good and useful.

I don't want to get too deeply into engineering detail but overload or even short-circuit protection can be difficult with high-current circuits on very low voltage (12V) supplies. Which is why vehicle manufacturers do not protect starter motor circuits.

Mind you, some offered isolators may not serve the requirement. A winch isolator needs to be capable of continuously handling 200 to 500 amps. The "Key Switches" on offer usually do not meet this. Jaycar advertise a "500A" switch but the fine print says "120A continuous" and Supercheap advertise a "1000A switch" with a fine print of only 100A continuous! Probably neither of these would reliably serve winch duty.

Probably the "screw-down" variety of isolator where a knob screws-down a bridge onto a contact surface would be best. (Illustration below) However even here quality would be an issue.

Image Could Not Be Found

I should point out that not having a winch, I have not tested such winch isolators so my comments are based on engineering theory only. It would be good to hear from those with experience in this.

Cheers
Allan

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