Aux lights - fuse or CB

[Glenn-BJ74]

Footnote.
Most automotive fuses are rated at X amps at 32 volts. So using a 10 amp fuse should open (sever) the circuit at 32 volts X 10 amps = 320 watts. In a 12 volt system the charging voltage is about 13.5 to 14.4 volts, so a 10 amp fuse will blow at between 22 to 24 amps at that voltage range. I believe fuses are rated at 32 volts so they can be used in 24 volt systems, which have charging voltages up to about 29.5 volts. Amp ratings are related to voltage - they are not independent

 

[SUMOTOY]

Footnote.
Most automotive fuses are rated at X amps at 32 volts. So using a 10 amp fuse should open (sever) the circuit at 32 volts X 10 amps = 320 watts. In a 12 volt system the charging voltage is about 13.5 to 14.4 volts, so a 10 amp fuse will blow at between 22 to 24 amps at that voltage range. I believe fuses are rated at 32 volts so they can be used in 24 volt systems, which have charging voltages up to about 29.5 volts. Amp ratings are related to voltage - they are not independent

Well yes and no regarding 'independent', the real question is this rating standard 'significant' or 'relevant'.... The math of the specifications for fuses isn't quite that simple, especially automotive applications. First, a typical Bussman ATC 10amp fuse @32v will never open at 10 amps, it will severe at 15amps minimum. Second, the conditions of the ATC fuse tests are done at Standard Temperature and Pressure (STP) with new wire, properly sized, connected and fused. Third, fuse to amperage load severs are not linear, they are time vs load. Bussman shows the 10amp (@ 32vDC) ATC fuse capable of loads exceeding 50amps for .1 second, and 175amp loads for .01 seconds. That curve doesn't come down to 15amp sever until 1.5 seconds. What does this all mean?

Cars operate under severe environments, and it must operate in real world installed conditions: wire age/size/connections, start-up load surge of motors and lighting, vehicle age, installed temps (engine bay and cycling interior temp extremes), voltage regulator, vibration, resistance, etc.... All these factors applied to the ATC fuse sizing since the 1970's, and appears a validate the same standard for Automotive application. Most auto applications put a 14.2v safety factor of 1.5 (non motor circuit ) to 2.0 (motor circuit) to the ATC fuse standard. 40 years has shown that to be a darn good target, even with new standard-system voltage regulation increasing from 12.7v to 14.2v during that time. We have 40 year old fuses installed, still blowing (severing) just fine when a circuit is overloaded, with no catastrophic failures.

Technically you are right, SAE J1284 puts ATC fuse ratings at xxAmps @ 32v @ STP. IMO, that shouldn't be a consideration for the 14.2v (re)calculation of ATC fuse applications. ATC fuse factor of 1.5 (rounded up) continuous load for automotive lighting is pretty standard for oe and aftermarket. It's also a good standard for relay applications as well.

HTH and not too nerdy

Scott J
94 FZJ80 Supercharged

 

[80t0ylc]

..................IMO/E an inline AGC fuse on aux lights is a basic wiring mistake, and a CB just doesn't seem any better in application to me. For point of reference, there isn't a single Hella Aux lamp/relay/harness that uses a CB instead of a ATC fuse. The two experts in the field of aux lights are:
Susquehanna MotorSports - High Performance Vehicle Lighting and Competition Accessories
Daniel Stern Lighting Consultancy and Supply

Neither recommend nor list a CB for Aux lights, for exactly the reasons I've proposed. It's not the cost, it's the fact that a standard issue weather protected ATC fuse is the best tool for the job of protecting lighting circuits. In my own 30 years of wiring/installing aux lighting, this is the first time I've seen or heard of using a CB in place of a ATC fuse. 'It works', but really tough to argue 'it's better'.

Cheers and my .02

Scott J..........................

So on 12/9/11 I contacted Daniel Stern Lighting Consultancy:

To products@danielsternlighting.com


Greetings:

I have a question about using an Automotive Circuit Breaker instead of a fuse. I understand there are at least 3 types: Type 1 is auto reset; Type 2 is reset by removing power from the circuit and then reapplying power; and Type 3 which has a button or switch to manually reset it, once it is tripped. I have heard that for Auxilary Light installs, that the Type 1 CB is a bad idea due to the potential for the CB's contacts to become "welded on" if the circuit experiences a short and no operator intervention occurs to shut down the lights. But is there any reason that you would discourage the use of Type 2 or Type 3 CBs in place of fuses in lighting circuits? Thank you in advance for any advice you may have on this matter.

Their reply today:

Hi, TB.
Sorry for the long delay in communication -- this is a
particularly rotten time of year to fall ill! You write:



> I have a question about using an Automotive Circuit Breaker
> instead of a fuse. I understand there are at least 3 types:
> Type 1 is auto reset; Type 2 is reset by removing power from
> the circuit and then reapplying power; and Type 3 which has
> a button or switch to manually reset it, once it is tripped.

Type 3 is fine. Neither Type 2 nor Type 1 should be used in
a lighting circuit. Lights do not draw moderately or
progressively excessive current as motors and other kinds of
loads can. Either the lighting circuit is fine, or it
dead-shorts. If it dead-shorts, you want that portion of the
circuit killed DEAD by the circuit protection device (fuse
or CB) until it is repaired. A Type 3 will carry on sending
pulses of power to the shorted circuit, and a Type 2 will
reactivate the circuit if the driver switches the lights off
and then on. Neither is desirable.

I believe there is a typo in the 2nd to the last sentence where they say "A Type 3 will carry on sending pulses of power to the shorted circuit, ...". I'm sure they meant to say Type 1.
Anyway, from this reply, I would conclude that if you choose to use a Type 3 CB instead of a fuse to protect your circuit, it will be "fine".

 

[John McVicker]

Tom,

Good update. Am learning from this thread. Not certain if DS is correct or not correct, but I agree w/your conclusion of his poorly worded response to you.

John

 

[tclark]

All this over a fuse for a little ole auxillary light--imagine if the op had asked about setting up dual batteries and an onboard welder or something like that!
Good reading guys, keep it coming.

 

[SUMOTOY]

So on 12/9/11 I contacted Daniel Stern Lighting Consultancy:
>...
I believe there is a typo in the 2nd to the last sentence where they say "A Type 3 will carry on sending pulses of power to the shorted circuit, ...". I'm sure they meant to say Type 1.
Anyway, from this reply, I would conclude that if you choose to use a Type 3 CB instead of a fuse to protect your circuit, it will be "fine".

Sure. I would agree with what Daniel Stern lighting meant to say, and did in post 17

My Conclusion: In the scenario above, I doubt you'd have any idea anything was wrong prior to catastrophic failure. Hence my point, the Auto-reset CB is not the proper circuit protection device for Aux lights. Specifically, the ATC fuse gives better protection to the primary directive. IF using a CB is a must-do, a Manual Reset Type 3 should be the minimum protection.

The type 1CB you used in your OP was not the correct application, neither is a type 2 CB. You *can* use a type 3 ATC Circuit Breaker, though I'm not sure I see any advantage to doing so. The main reason for using the ATC fuse with weatherproof housing in place of the inline fuse you used? The Type 3 ATC Circuit Breaker will not fit in a standard ATC weather protected fuse-housing (The type 3 CB is taller). And, AFAIK, I have never seen a weather protected housing for a Type 3 CB. It is meant to be installed in-cabin or in a weather protected fuse block that can accommodate the extra height of the type 3 CB.

That sounds like a lot of work to avoid a 40 cent ATC fuse

Cheers

Scott J
94 FZJ80 Supercharged
ATC weather protected fuses aplenty