Placement of Fuse in Circuit

rampantbunny

Hi all - I've deliberated over the placement of this post so if a mod thinks it's better somewhere else, feel free to relocate.
It's an electrical question, albeit not actually Home and Garden, more automotive, but I imagine there are more sparks monitoring this forum.

Q1. I've run a cable from a battery positive to an ammeter then on to a switch (ignition). Should a fuse be inserted along this line somewhere? (Will call this Fuse1).


From the single output terminal of the ignition switch, power is routed to a fuse box, and in turn to various secondary circuits for items such as headlights, indicators etc...
Depending on the answer to Q1, I may already have a Fuse1 in the circuit either before the power comes into the ammeter or the ignition switch...therefore if I route to a fuse box I'm effectively doubling up on fuses for all the secondary circuits i.e. there'll be Fuse1, and whatever fuse is required for the secondary circuit.

Q2. As everything that will draw current from the battery will be routed through this initial fuse, wouldn't I have to calculate the combined amperage of all the possible electrical items that could be switched on at the same time, in order to work out what amperage the first fuse should be? i.e. it will have to be rated large enough to cope with the combined current draw of multiple items?

Q3. Lets say the above calculation is 50A; is there even any benefit in having the fuse there in the first place i.e. will it provide any protection to the ammeter\ignition switch if rated this highly?

Bruthal

rampantbunny wrote: »

Hi all - I've deliberated over the placement of this post so if a mod thinks it's better somewhere else, feel free to relocate.
It's an electrical question, albeit not actually Home and Garden, more automotive, but I imagine there are more sparks monitoring this forum.

Q1. I've run a cable from a battery positive to an ammeter then on to a switch (ignition). Should a fuse be inserted along this line somewhere?

Q2. From a single terminal on the ignition switch, power is distributed to a fuse box, and in turn to various secondary circuits for items such as headlights, indicators etc..

I get confused here
Depending on the answer to Q1, I may already have a fuse in the circuit either before the power comes into the ammeter or the ignition switch...therefore if I distribute to a fuse box I'm effectively doubling up on fuses.

And as everything that will draw current from the battery will be routed through this initial fuse, wouldn't I have to calculate the combined amperage of all the possible electrical items that could be switched on at the same time, in order to work out what amperage the first fuse should be? i.e. it will have to be rated large enough to cope with the combined current draw of multiple items?

Q3. Lets say the above calculation is 50A; is there even any benefit in having the fuse there in the first place i.e. will it provide any protection to the ammeter\ignition switch if rated this highly?

Most circuits in a car should have a fuse. An example of one that does not is the starter motor supply, because it operates at a level higher than is practical to fuse, and the main battery leads are large to accomodate this current.

For your question 3, the reason having a main fuse on a cable such as your asking about, is to prevent the cable itself going on fire, so an adequate fuse calculated by the method you suggested is best.

Make sure the cable in question is large enough to carry the current required. It sounds like you might be better supplying the fuse box first, then the ignition, as the ignition switch only needs a small cable/fuse supplying it. Then use relays from the outgoing of the ignition to allow devices fed from the fusebox to only operate then the ignition is on.

paddymick

rampantbunny wrote: »

Q1. I've run a cable from a battery positive to an ammeter then on to a switch (ignition). Should a fuse be inserted along this line somewhere? (Will call this Fuse1).

Yes you should have a fuse located as close to the power source as possible,this should be big enough to cover the combined load of everything and some.
This fuse is there mainly to protect the wire feeding your fusebox which must also be big enough to cope with the full load..
If every individual circuit is wired and fused correctly then this main fuse in theory should never blow.

From the single output terminal of the ignition switch, power is routed to a fuse box, and in turn to various secondary circuits for items such as headlights, indicators etc...
Depending on the answer to Q1, I may already have a Fuse1 in the circuit either before the power comes into the ammeter or the ignition switch...therefore if I route to a fuse box I'm effectively doubling up on fuses for all the secondary circuits i.e. there'll be Fuse1, and whatever fuse is required for the secondary circuit.

The fuses in the fusebox will be smaller and will only cover its dedicated circuit, this allows you to use a smaller wire for each individual circuit other wise you would have to wire them using the same size wire as your main feed which just wouldnt be practical..

Q2. As everything that will draw current from the battery will be routed through this initial fuse, wouldn't I have to calculate the combined amperage of all the possible electrical items that could be switched on at the same time, in order to work out what amperage the first fuse should be? i.e. it will have to be rated large enough to cope with the combined current draw of multiple items?

Yes you would need the combined load and then add a little to give your self some play.

Q3. Lets say the above calculation is 50A; is there even any benefit in having the fuse there in the first place i.e. will it provide any protection to the ammeter\ignition switch if rated this highly?

You have to have the main fuse in order to protect the main feed in case of overloading.
You would need to know the ratings for the ammeter and switch in order to know if they can take the load if not relays will be needed.

Also if you are doing work on car electrics I find haynes manuals great for circuit diagrams..

rampantbunny

Hi Robbie7730, thanks for the very quick reply.

A very heavy gauge cable runs from the battery directly to the starter motor - so yes, the starter motor supply is not fused.The cable I'm enquiring about is the cable that runs into the "cab" which is a heavyish gauge cable from the battery

My current set-up is: heavyish gauge battery live cable runs to ammeter, then onto ignition switch, then onto the starter switch, then to the starter solenoid\relay.
This is not the main starter "supply". The cable is heavy duty enough to supply the needs of all combined secondary circuits so no problem there.
If I were to insert a fuse into this circuit, I'd run the cable to the fuse box first, then to the ammeter, then ignition switch, then starter button etc..

I've used some relays already in my secondary circuits where I have:
Directly from ign. Live, I run a smaller smaller gauge wire to e.g. a combined Park\Dims\Headlight switch.
I also run an unfused ign. Live to the main input of the relays.The output of the switch switches the relays (of course). The relay output is then goes to a fuse, then to the electrical items.
I did it this way as it allows me to have individual switch for each output of the parks\dims\headlight switch. If I ran a fused ign. Live into the switch it would mean a single fuse for my parks\dims\heads - this make sense?

Also, if I do what you've suggested i.e. use a relay on the output of the ignition, this would mean the relay being used to feed switches (with fuse probably before the switch)..and the switch outputs wired directly to the elec. item. But the switch is now handling more current than it possibly should if I wire in this fashion - correct?

So many choices :eek:

rampantbunny

Nice one PaddyMick...I suppose my ultimate issue is wondering if this doubling up of fuses was in some way 'bad practice'.

And, if you read my response to Robbie7730, you'll see I also wonder mightily whether I should place fuses before or after the input to switches and relays.

Bruthal

rampantbunny wrote: »

Nice one PaddyMick...I suppose my ultimate issue is wondering if this doubling up of fuses was in some way 'bad practice'.

And, if you read my response to Robbie7730, you'll see I also wonder mightily whether I should place fuses before or after the input to switches and relays.

Fuses should be before inputs to switches and relays in general. And the main fuse for the supply to the ammeter and ignition switch should be as close to the battery as possible.

rampantbunny

robbie7730 wrote: »

And the main fuse for the supply to the ammeter and ignition switch should be as close to the battery as possible.

So for the ammeter circuit, to get as close to the battery as possible I'd have to get something like an in-line fuse?

robbie7730 wrote: »

Fuses should be before inputs to switches and relays in general.

Are you saying I should put a fuse before my switch(s)? Or are you saying what I've done will work but is not doing it by the book?

Because I'm using relays to handle the heavier current, my switches are not exactly being loaded, so are fuses on the input to the switches really necessary?
If yes, I'll need fuses for every switch input and if there are multiple outputs from the switch, I'll need fuses on those too.

Bruthal

rampantbunny wrote: »

So for the ammeter circuit, to get as close to the battery as possible I'd have to get something like an in-line fuse?

That would be one way, but blade fuse holders can be got as well, and mounted near the battery.

Are you saying I should put a fuse before my switch(s)? Or are you saying what I've done will work but is not doing it by the book?

Because I'm using relays to handle the heavier current, my switches are not exactly being loaded, so are fuses on the input to the switches really necessary?
If yes, I'll need fuses for every switch input and if there are multiple outputs from the switch, I'll need fuses on those too.

The main supply should go into the fuse box, through circuit fuses and out to the relays contacts and on then to the relay controlled circuits, and also switches of direct switched devices, if any, should also have their fuses before the switches. So basically the main supply to the fuse box should be straight into the line of fuses before anything else.

So the low load switches (which switch the relays for the bigger loads) should be themselves fed through a small fuse, so the relay switching control circuit will be fused also.

rampantbunny

robbie7730 wrote: »

That would be one way, but blade fuse holders can be got as well, and mounted near the battery.

Was actually thinking of getting something like this:
http://www.ebay.co.uk/itm/Waterproof-Heavy-Duty-Maxi-Blade-Fuse-Holder-inline-/110753346092#ht_523wt_1235
My main fuse box is located a distance away from the battery. With the above I'd be able to get a fuse much closer to the battery plus I only want a single fuse.

robbie7730 wrote: »

The main supply should go into the fuse box, through circuit fuses and out to the relays contacts and on then to the relay controlled circuits, and also switches of direct switched devices, if any, should also have their fuses before the switches. So basically the main supply to the fuse box should be straight into the line of fuses before anything else.

So the low load switches (which switch the relays for the bigger loads) should be themselves fed through a small fuse, so the relay switching control circuit will be fused also.

Got it Robbie7730:
Small fuses required on the low load input for switches that switch relays.
Fuses on the relay inputs.
Fuses on the input to direct switches too.

Many thanks.

paddymick

Just out of curiosity what are you actually doing?
Is it a scratch built project or are you reshelling a car or something?

rampantbunny

paddymick wrote: »

Just out of curiosity what are you actually doing?
Is it a scratch built project or are you reshelling a car or something?

...well, although I did say automotive it's not actually a car.
It's an old tractor that has long since lost all ability to illuminate the darkness. I mean, no parking lights, dims, heads, indicators, horn..the list goes on.
Very little in the way of electrics are working, and wires seem to start in mid-air and finish there.

Robbie7730s advice is much appreciated, but I realise that I'll end up with more fuses than were in the original manufacturers design. How bad though...there'll be no fires anyway

paddymick

rampantbunny wrote: »

It's an old tractor that has long since lost all ability to illuminate the darkness.

An aul tractor sure why didnt you say,
Forget all that fancy fuses and switches stuff and just run the wires straight to the battery from the lights and when you want them on just twist the bare ends around the terminals she`ll be grand, Its the farmers way:D:D.

Ah im just kidding good luck with it:)

Bruthal

rampantbunny wrote: »

...well, although I did say automotive it's not actually a car.
It's an old tractor that has long since lost all ability to illuminate the darkness. I mean, no parking lights, dims, heads, indicators, horn..the list goes on.
Very little in the way of electrics are working, and wires seem to start in mid-air and finish there.

Robbie7730s advice is much appreciated, but I realise that I'll end up with more fuses than were in the original manufacturers design. How bad though...there'll be no fires anyway

Well you might not need fuses for each individual circuit. On cars, seperate fused circuits would be used for left lights and right, low and high beam etc, for safety of not losing all lights with a blown fuse, but other services often are shared with a fuse for a couple of items.

Bruthal

rampantbunny wrote: »

Was actually thinking of getting something like this:
http://www.ebay.co.uk/itm/Waterproof-Heavy-Duty-Maxi-Blade-Fuse-Holder-inline-/110753346092#ht_523wt_1235
My main fuse box is located a distance away from the battery. With the above I'd be able to get a fuse much closer to the battery plus I only want a single fuse.


Got it Robbie7730:
Small fuses required on the low load input for switches that switch relays.
Fuses on the relay inputs.
Fuses on the input to direct switches too.

Many thanks.

Yes thats about it.

rampantbunny

paddymick wrote: »

An aul tractor sure why didnt you say,
Forget all that fancy fuses and switches stuff and just run the wires straight to the battery from the lights and when you want them on just twist the bare ends around the terminals she`ll be grand, Its the farmers way:D:D.

Haha...well up to now we had candles up front, but they kept blowing out. If I could find a solution to that I'd save a fortune on wiring :rolleyes: