Simple electronics

OK, easy (I expect) question…

In a 12V DC power cable to a piece of electrical equipment, why would the supplier include a separate in-line fuse in both the +ve and -ve leads? I have one in front of me and need to carve it up (for unimportant reasons), and I am wondering whether I need to include the same sort of two-fuse arrangement in the new lead.

As far as I can see, the purpose of the fuse is to protect the equipment from a short circuit in the kit, by blowing the fuse when the current ramps up, yes? So, surely one fuse anywhere in the circuit would suffice? With two fuses, which would blow first, and why?

And also… Sorry to ask 2 questions in one post, but… In a boat, running a generator to supply mains voltage, what should I do about earthing the circuit? I have read contradictory advice on this, with some suggestions of an earthing connection to the water, some suggesting it’s safer without… I know this is the place for the full, accurate and definitive answer!

Is it a +/- 12 VDC supply (with a third “common” lead), or just a regular 2-wire 12 VDC supply?

For the latter, I usually just stick a fuse on the +12 V lead. Unless there’s a voltage between the common lead and earth ground (or the chassis), then there’s usually not a reason to fuse both. And least I think I’m right about this…

Excellent… an answer… thanks :smiley:

It is a regular 2-wire supply, no common or ground lead.

You need two fuses cos there is a voltage between the negative voltage and ground. ie a current will flow from ground to the negative terminal. Just because its negative doesn’t mean it can’t do damage.

Don’t be sorry. You answered both questions with the other question.

A less common reason, which applies to highly inductive loads, like large motors, expecially with flywheels, is the possibility that the inductive kickback after shutdown can send high voltage transients back up the line and damage the supply (the usual bypass diode isn’t always enough in damp environments or if there is arcing). This could also occur in the event of failures in devices which step up the voltage.

A friend once dropped a night vision scope that was plugged into his car cigarette lighter. I can’t recount the exact details, because he was neither completely forthcoming, nor very civil, after the long walk home

I don’t know what kind of equipment you are using, but a fuse is there to protect against current (amperage). If you ground something, the voltage will stay the same but your voltage will (for the purposes of imagination) be through the roof.

Your fuse should have a label or some sort of marking on it. I’m guessing either you have either an a) sensitive piece of equipment, or b) something that uses a lot of current. In the case of the latter (such as a washer or dryer), you might want to see if your circuit breaker can handle the load.

I think this is the first time in years I’ve used my degree. Wow.

One of those "voltage"s should read “current”. I’ll leave it to you to figure out which one, since I expect you know anyway. :slight_smile:


Yer totally right. It should read “. . . but your current will be through the roof.”. :smack:

Damn keyboard. :mad:

Personally, I blame the Communists.

OK, I understand some of the posts (:smiley: )… Thanks for them. I am still unclear as to WHY the -ve lead needs protecting with a fuse though…

Munster said :
“You need two fuses cos there is a voltage between the negative voltage and ground. ie a current will flow from ground to the negative terminal. Just because its negative doesn’t mean it can’t do damage.”
I don’t get this… obviously my understanding of electronics is poor… please explain? Surely, in the event of a grounding or short somewhere in the equipment or the power lead, a fuse in the +ve lead would blow, whatever? I can’t see what added benefit the fuse in the -ve lead gives.

AcidKid posted:
“Don’t be sorry. You answered both questions with the other question.”
No, don’t get that at all, lol The two questions were (I thought) unrelated… the first is re the fuse in the -ve lead, the second was about earthing electric circuits in boats… perhaps I should have posted em separately. I’ll stick to question one.

KP - Good answer, but in this case the equipment is very low load (~1A) no moving parts, purely electronic in nature. Power supplu is also very cheap (simple lead acid battery), whilst the equipment is very expensive. It IS intended to be used outdoors though, could this be relevant?

Tripler - Thanks for the reply… the equipment is valuable ($30,000 ish), but low current - see above.

If the answer to my OP is in the replies so far, I can’t see it… don’t mean it isn’t there though!

Sorry I was unclear.

My point was if the +ve fuse is blown there is still a route for the current to flow, ie from groung to -ve.

If there is a short in the system without the -ve fuse a large current could still be drawn hence damaging whatever your trying to protect.

With the -ve and +ve fuses, if there is a short both fuses will blow so there is no path for the current to flow.

My bet is that the cables were supplied with a piece of equipment that could be installed in either a positive or negative grounded environment.

Unless the power generating unit produces positive and negative voltages, overcurrent protection on the line being grounded or bonded is of no benefit.

In the event of expensive equipment, always use a fuse. They are available in many flavors: slow-acting, fast-acting, and so on, but they are a simpler device than an inverse-time-element circuit breaker.

As for grounding the output of a 120/240V AC inverter…

I’m not an expert, but I think the normal reasons for grounding do apply on a boat.

In a home environment, if a 10KV AC power line blows down and falls across the 120/240 lead-in wires outside the house, then what will happen? If one side of the 120/240 circuit is grounded, then the 10KV line will be shorted to earth, and breakers in the substation will blow, disconnecting the 10KV.

But if both sides of the 120/240 circuit are floating, then the whole circuit inside the house will operate as normal, yet both wires will “float” at 10KV AC with respect to ground. This is a very lethal hazard, obviously. So, we ground the circuit inside the house to protect the occupants from high voltage power lines which wander around during a storm, earthquake, etc. No point to grounding it on a boat then?

But there’s a second problem. During a lightning strike, if your 120VAC circuit is floating, and if lightning strikes part of the circuit, then as the lightning is seeking out a path to ground, the whole circuit can launch big arcs to any nearby conductor (such as people who are holding an appliance, a radio microphone, etc.) On the other hand, if one side of the AC circuit is grounded, then chances are that the main current will easily get to ground without having to leap through any air-gaps which happen to have human bodies in the way. Ground one side of the 120V circuit in order to allow the circuit to become part of the boat’s “lightning rod” system. After all, sometimes lightning comes in from the side, and can miss the mast and strike some part of the superstructure.