Thread: circuit breaker or Maxi fuse?

Originally Posted by WildDisco

What about the likely hood of them welding up as that is the main point I hear circuit breakers bagged for?

Hi Ben, sorry to have taken so long to get back to you, I’m still getting over a trip to Melbourne and back.

Next, bellow is some data I will eventually be putting on my web site and although not completed, it will give you some guide lines to work from.

As to Circuit breakers welding up, I have never come across that situation but, as harry has pointed out, using good quality circuit breakers is imperative to making your circuit as safe as possible.


GENERAL CIRCUIT BREAKER INFO

When selecting the size of cable, bigger is usually better, however, the exact reverse is the case when selecting the correct size fuse or circuit breaker.

Many people make the potentially lethal mistake of installing too high a current rated fuse or circuit breaker. The thinking that, if the cable can carry 100 amps then the protection should be either a 100 amp fuse or circuit breaker is a BIG mistake.

When designing high current circuits, for a starter, use circuit breakers, as these are much safer because circuit breakers have a better response time to higher currents than fuses do.

In either case, something like a 100 amp fuse or circuit breaker is designed to be able to handle around 150 amps for nearly 1 hour before blowing or tripping and can handle something like 400+ amps for a few seconds before tripping.

This high current handling is deliberately designed into these devices to allow them to handle the split second near dead short that occurs every time an electric motor starts, IE like your fridge’s compressor motor and your portable air compressor’s motor and the split second high current surge that occurs when you turn your lights on. Some driving light globes can cause as much as a 100 amp spike when they are turned on.

In most dual battery set-ups, no matter what type of Auxiliary battery is being used, a 30 amp circuit breaker will be quiet adequate.

In engine bay installations where there is a short wire run, one circuit breaker, anywhere along the positive cable will do, but where the auxiliary battery is mounted somewhere else, IE in the rear of the vehicle or in a trailer or caravan, the you need to mounted a 30 amp circuit breaker at either end of the positive cable, near each battery’s positive terminal.

Using this sort of set-up not only gives you much safer protection in a dead short situation because the 30 amp circuit breaker set-up will not only trip much MUCH quicker ( the 100 amp will probably work but may actually take a few seconds to go open circuit ), the 30 circuit breaker set-up also gives you overload protection. The 100 amp set-up in an overload situation can actually allow the cables to melt and still not go open circuit.

Thanks drivesafe.

One last question, how durable are circuit breakers? As in under the bonnet is it a problem getting wet and muddy so a cover should be used.

Originally Posted by WildDisco

is it a problem getting wet and muddy so a cover should be used.

Back to harry’s post, get good quality ones and you won’t have a problem.

I sell Cole Hersee circuit breakers. They are probably the Rolls Royce of circuit breakers and they are priced accordingly.

So far I have never had one that had had any problems with water and these devices are fitted in all sorts of under bonnet locations and quite a few have done the preverbal river crossing or two and none and to my knowledge none have had any form of cover put over them and they still work fine..

Cheers

Good quality magnetic circuit breakers are very, very reliable. In marine science we won't use fuses offshore at all. In a similar way to desert operations there are no spare parts dealers around at sea. Many of the breakers usually found in 240V mains distribution boards are also recommended by the manufacturers, e.g. Hager, as being suitable for low voltages as well. They break upon a current overload (AMPS) so the operating voltage in a 4WD being 12V DC as opposed to 240V AC doen't matter as long as the manufacturer specifies that they will do the job. Head for an electrical wholesaler. Trade prices are from about $10.00 ea.

Originally Posted by Dunnie

Many of the breakers usually found in 240V mains distribution boards are also recommended by the manufacturers, e.g. Hager, as being suitable for low voltages as well.

Just make doubly sure that the 240v version is marked as suitable for DC use because normal AC devices will not tolerate the high current arcing that occurs with DC, something that does not have to be catered for with AC operations.

I have designed my own intelligent breakers for the formula SAE car. Combination of polyfuses, 555s and opamps make a beautiful current monitor. And can be reset simply by turning the circuit off and back on. Will probably look into a micro controlledversion soon, but that can wait till after decembers racing.

Cheers!

Ive seen a circut breaker weld itself into the closed circut position exactly once in about the 20 years that Ive been fiddling with mechanicing.. IT was one of those cheap self reseting thermal type of circut breakers, they are also known as self reseting circut breakers.

as they work on exactly the same principal as your flasher can for the indicators they warm up a little when they are working, especially at the higher amps end of the scale, If you get a big enough short in a big enough circut and ignore it for long enough the chance of them fusing goes up. Once they fuse thats it, game over the power stays on until the bimetalic strip tears itself off of the contact or something else burns out.

Go circut breakers as harry said Dont go cheap ones, I like the magnetic latch types and even the thermal latch types are ok for short protection (but dont rely on them for component protection the instant trip amps is something like 3x the rated current) and I'll go against Deejay on the fuse holder circut breakers as the good ones are good things, If they use them to protect the circuts in airfield maintenance gear, AFV's and bushmasters they get my stamp of approval.

The ones Im using at work let go at 125% of rated in less than a second, have a test switch which lets you "pop" the breaker without removing it, has a push to reset button that doubles as an indicator as to its condition, they fit in the place of any normal blade fuse holder providing it has about 15mm of clearance over the top to accomodate the height of the thing. Ive found a couple of places that have some on stock and a couple that will order them in if needed.

Originally Posted by Blknight.aus

The ones Im using at work let go at 125% of rated in less than a second, have a test switch which lets you "pop" the breaker without removing it, has a push to reset button that doubles as an indicator as to its condition, they fit in the place of any normal blade fuse holder providing it has about 15mm of clearance over the top to accomodate the height of the thing.

Hi Dave, fast blow fuses and circuit breakers that have a low current surge tolerance, such as the type you described are not very suited to automotive use.

These types of devises are designed to trip at the slightest increase in current, which may sound like the ideal form of protection but many auto electrical circuits will regularly carry high current surges as a normal operation.

The most common is any circuit that is powering a motor of some form. Electric motors often need such a start-up current that the circuit goes to a near dead short situation and if you wanted to use circuit breakers that have a low current surge tolerance, you would have to fit a much MUCH higher current rated one to make sure it doesn’t trip every time your motor turned on and this increase in current rating of the fuse now means a dramatic increase in the size of the cable is needed, just to use the wrong fuse for the job required.

Up to 30 amps, standard automotive fuses are best suited for the job they are designed for.

Over 30 amps, then the circuits operation has to be taken into account when deciding on the type of protection to be used.

When designing a dual battery set up, you have to consider both the normal current requirements and the high current start-up requirements. Most single battery DBS require about 15 to 25 amps while charging a battery. If the battery is fairly low when starting up, the battery can easily pull 40 to 50 amps and some AGM can pull 100+ amp for a few minutes.

Most AGM batteries CAN pull these high currents but the CAN NOT tolerate this form of current for anything but a short time without causing long term damage to the battery.

By using an auto resetting type circuit breaker, you can not only protect against shorts but the circuit breaker can actually work as a regulator by cutting in and out while the AGM battery tries to pull the high initial charging current.

Having the circuit breaker act as a current regulator spreads the effects of the initial high charging requirements out over a longer period of time, protecting the battery and the set up.

Driving lights can also require a knowledge of their operation to be able design the proper circuit for them.

You need to have a fair idea of how a circuit is going to operate before you start deciding the type of circuit protection that best suites your needs.

Cheers.