TC-33 electrical nightmare ENDS ! HAPPILY !

[jinman]

This is a common error. The power dissipated in a given electrical circuit does not remain constant when resistance changes affect the circuit. In this case, the overall total voltage drop remains constant, the the total amps drop and therefore the total power drops. There is just added voltage dropped across the resistance, leaving less to drop across the original load.

Guy.

Guy, I think we are on the same wavelength here.

 

[jwstewar]

I'm not an electrician and don't know much about the "rules" of electric so I hate to argue with you Jim, but I've seen a bad ground blow fuses. It was on my Dad's 78 Chevy pickup. Lights worked fine. Hooked a camper to it and started out w/ lights all working. The fuse for the parking lights immediately blew and so did the fuse for the turn signals. What did we fix? The ground wire for the trailer plug had loosened up and caused corrosion. I took the wire down cleaned the connection and the frame, put a grounding washer on it and replaced the fuses. All worked fine from that point on until they got rid of the truck about 10 years later.

 

[SteveInMD]

I agree with the above too.

However, whenever my tractor sits for more than about 10 days I will often blow the 10 amps fuse in the safety interlock & starting pinon circut. If I charge the battery first (or have run the tractor recently) then I never blow the fuse. The battery is not bad - it tests strong. If battery is completely topped up I'm fine. If not there is a good change the fuse will blow.

Of course my solution is not never go more than a few days without some tractor time!

 

[guyw]

I'm not an electrician and don't know much about the "rules" of electric so I hate to argue with you Jim, but I've seen a bad ground blow fuses. It was on my Dad's 78 Chevy pickup. Lights worked fine. Hooked a camper to it and started out w/ lights all working. The fuse for the parking lights immediately blew and so did the fuse for the turn signals. What did we fix? The ground wire for the trailer plug had loosened up and caused corrosion. I took the wire down cleaned the connection and the frame, put a grounding washer on it and replaced the fuses. All worked fine from that point on until they got rid of the truck about 10 years later.

But in this case you changed the equation be adding the extra load of the trailer. This lowers the resistance of the circuit, allowing a greater current flow through the fuse. Might have been an old fuse ready to blow or something else came in to play. Could have been coincidence.

Guy.

 

[guyw]

I agree with the above too.

However, whenever my tractor sits for more than about 10 days I will often blow the 10 amps fuse in the safety interlock & starting pinon circut. If I charge the battery first (or have run the tractor recently) then I never blow the fuse. The battery is not bad - it tests strong. If battery is completely topped up I'm fine. If not there is a good change the fuse will blow.

Of course my solution is not never go more than a few days without some tractor time!

In the case of a lower voltage trying to engage a starter which is in effect a motor, you have to consider the inrush current in order to get the electric motor spinning. The initial startup current for a motor is much greater than the current required to keep it spinning. With a lower initial voltage, the startup takes longer and the time over which that larger current is flowing increases. This in turn increases the time that the fuse is heating up due to the current flow, and a fuse that would otherwise not blow at the higher voltage will now blow due to the lower battery voltage. In the case of a bad ground connection, the added resistance in the circuit limits the amount of current as well as reducing the voltage at the starter, making it a different situation.

Guy.

 

[jwstewar]

But in this case you changed the equation be adding the extra load of the trailer. This lowers the resistance of the circuit, allowing a greater current flow through the fuse. Might have been an old fuse ready to blow or something else came in to play. Could have been coincidence.

Guy.

Nope, because I replaced the fuses before I fixed the ground and they blew too. This took several hours of tracking stuff down and checking things like bulbs & plugs & sockets. All were good and nothing else ever got changed. You can say what you guys want, but I know what fixed the problem - because I'm the one that fixed it.

 

[jinman]

I'm not an electrician and don't know much about the "rules" of electric so I hate to argue with you Jim, but I've seen a bad ground blow fuses.

Jim, you aren't arguing with me, you are just not accepting that Ohm's Law is a law. What you did to "fix" the problem was not really the source of the problem. It contributed to a blown fuse, but the problem was a circuit that pulled more current than the rated value of the fuse. If you had increased the value of your fuse, the problem would have gone away too.

When cold (off) a lamp's filament has less resistance than it does when it is on. As the filament becomes hot, the resistance rises. There is an initial surge of current that reduces as the filament heats. The properly fused circuit will have fuses well above the maximum draw of the lamps when cold.

Some fuses are called slo-blow type because they can take short surges of current above their rated value. A continuous current above their rated value causes them to blow.

If you have a lighting circuit that pulls current at almost the fuses limit when hot, what do you think will happen if you repeatedly make and break contact? What happens is the average surge current through the lamp's filament exceeds the fuses maximum rating and it finally blows. This is most often seen as a delayed reaction.

I think when you added the trailer, you were right at the limit of the fuse. Because of the intermittent ground contact, the lamps never heated up and their resistance never rose to the rated value when on full bright. As a result, their lower resistance pulled too much current. By design, a bigger fuse was needed. If you had never added a trailer, bad connections on the truck would never have caused this problem.

So, I'm saying that what you did had the effect of not blowing the fuse, but that same fuse was not very "happy" when you added the trailer, not even the new one after you fixed the ground. You were always just a few milliamps away from another blown fuse.

 

[jwstewar]

OK, I understand now. But just to say a bad ground won't cause a blown fuse, while technically correct isn't necessarily true either. How is that for a confusing statement? Because in my case the fuse was the proper size, but because of the ground issue it caused the load to pull more amps. Why did it pull more amps, because of a bad ground.

 

[guyw]

OK, I understand now. But just to say a bad ground won't cause a blown fuse, while technically correct isn't necessarily true either. How is that for a confusing statement? Because in my case the fuse was the proper size, but because of the ground issue it caused the load to pull more amps. Why did it pull more amps, because of a bad ground.

I think the problem is that to paraphrase the initial statement, Ohm's law says that because the resistance in the circuit increased, the power increased. That is what Jiman and I both took issue with. Ohm's law and the power equation actually say just the opposite. The rest has been trying to explain why you and others are seeing what you see with a bad ground connection. Bad grounds cause all kinds of bad things including radio frequency interference from electronic equipment. When you have a load that has an initial inrush current, like a motor or a light bulb, even though the absolute peak current will be less in the circuit due to the increased resistance of the bad ground connection, the duration of the peak or near peak current draw will last longer as the voltage and current starved load tries to come up to operating temperature or operating speed. Because of this the fuse in the circuit sees the initial inrush current for a longer time, allowing it to heat up and blow. Initial inrush currents for motors and light bulbs can be tens if not hundreds of amps for a circuit that under normal steady state operating conditions would draw less than 10 amps.

Guy.

 

[SteveInMD]

So, that would explain why dirty safety interlock switches could cause the starter solenoid to blow a fuse.