# Can a low voltage situation cause a fuse to blow?

1. Aug 2, 2015

Hello all. First, let me say that have very little education in the electronics field, but I do know the basics.

I recently got hired as a service tech for pressure washers and had encountered a unit that had a small engine (electric start 12v) with a blown fuse.

A co-worker of mine has been doing this work for about 15 years. He is the know it all type. Very good with wiring and troubleshooting electrical problems with the bigger electrical units (120v - 460v), however he told me that the reason this machine I was working on blew a fuse because the battery was low(which it was dead when it came in).

I explained to him that according to my understanding of ohms law, that doesn't make sense. If the voltage was low the amperage should have dropped too. Is this correct?

He told me that he has seen when low voltage had caused a fuse to blow or breaker to trip (in high voltage machines).

Is there any situation at all that this would be true? It is really making me curious.

Thanks for any insight you can help provide.

2. Aug 2, 2015

### Hesch

It depends on the characteristic of the load (torque needed as for driving the pump).

Say you have a pump with a piston, thus the motor (supplied with low voltage) cannot start the pump.

You are right, using ohms law: I = V / R, but when the motor is stucked, it will not yield a counter-emk. So when the motor is stucked, you must use:

Istucked = Vsupplied / R

and if the motor can start, you must use:

Irunning = ( Vsupplied - Vcounter-emk ) / R

It may happen that Istucked > Irunning.

3. Aug 2, 2015

### William White

Many rotating machines will be protected by a low voltage trip for safety reasons.

what you don't want is a power cut to stop the machine, then somebody sticking their fingers in there to check why it has stopped, only for it to start suddenly when the power is restored.

so in those cases, a manual reset of the machine and the breaker is required after the power is restored.

4. Aug 2, 2015

### jim hardy

that's a safe bet. DC motor's stall current should be brief enough a slow blow fuse will carry it, even if it's maybe 10X motor's continuous rating which is plausible.

but if battery can only deliver , let's just say 3x, motor wont get enough oomph to start the gas engine and fuse will blow.

5. Aug 2, 2015

Ok, I think I understand what you're saying as far as an electric unit causing these circumstances if there was a failure possibly in the pumps unloader ( pressure regulator).

However, for a gasoline powered unit, the only DC motor would be in the starter motor itself. You're saying that a temporary failure in the starter could have caused a high enough amp draw to blow the fuse?

I ended up replacing the fuse and charged the battery and haven't had any issues with it returning. But still not sure why it happend.

6. Aug 2, 2015

### jim hardy

i thought we were speaking of the starting motor on a small gas engine.
I'm not accustomed to starting motors being fused, but what do i know about your pressure washer ?

Look it over carefully for a wire that's nicked or has been rubbing from vibration and developed a bald spot.

We could speculate all day... can you measure battery voltage while cranking ? Rule of thumb : it should stay higher than nine volts.

7. Aug 2, 2015

### rootone

Fuses can blow just because they are old.
Continuous heating and recooling can change the properties of the wire so that it will eventually 'blow' at a lower current then rated for.
(In the same way that an incandescent bulb eventually fails)

8. Aug 3, 2015

### Jeff Rosenbury

Loads are sometimes of a set power level. For example a motor may be controlled to operate drawing 1kW (due to speed and torque considerations). Power is equal to the voltage time the current, so dropping the voltage will cause the current to increase.

On a large scale, things like refrigerators need to keep things cool. If the voltage drops, they run longer. If feeding thousands of homes, all of them can run all the time drawing more current.

Utilities tend to calculate loads by power, not voltage levels. Thus when the voltage drops, the current rises.