# TOO MANY FUSES?

#### THUNDERBOLT

Aug 29, 2010
22
Could someone please let me know if I am going overboard with the fuses.
(Plus any other comments on this circuit design).
This is really the first circuit I have designed using voltages higher than 12v, and using AC, so I am sure it is very much less than ideal.

I know there are no earth wires included, basically because I'm not too sure how many I need. For example does the charging transformer need its own independent earth also?

But really, my main concern was that I might be over cautious when it comes to fuses. I am just trying to cover each scenario that could lead to a fire.
I am envisaging a Lithium battery, so as yet I have no idea of the charging current, hence the two RED fuse values still need to be finalised, once the appropriate charger is selected.

I could start an explain my reasoning for each of the fuses and their values, but maybe its just easier if someone questions a particular fuse then I can try to explain that fuse in particular and my reason for including it.

There a few in there which I think might be surplus, but it would be good to have it confirmed by someone more experienced than me.

The whole package is to perform a certain function on a momentary basis (i.e. about 2 seconds every hour), but also sometimes needs to be used remotely, i.e. away from any power source, hence the 24v battery.

Most of the time however it can be used by means of the AC supply from the mains.

So, just one other question. Since the greatest fuse value is 10A would it be sufficient to use wire throughout, which is rated at 10A continuous?

Thanks for anyone who has taken the time to read this far.

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#### Militoy

Aug 24, 2010
180
To my way of thinking - you're overfused. A fuse is used as a deliberate "weak link", designed to fail safely, and prevent fire in the event of catastrophic failure of a component. I typically put a fuse in the very front end (where you have one) - but I place one in both lines, if there is any chance at all line and neutral could be reversed at the source. I also place one in the output circuit. In your case, I get the additional fuse in the battery charging circuit - as long as your charger is current-limited to prevent nuisance fuse tripping. When deciding to add additional fusing - remember you are adding natural failure points - and ultimately may be lowering overall reliability.

EDIT - Forgot to address the wire question - Wire current rating can be a bit subjective, and is usually based on some limit of temperature rise in free air, as well as bundled. You want your fuse to fail well before your wire or circuit board traces heat up. I would tend to err on the side of oversizing the wire, as opposed to undersizing it.

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#### THUNDERBOLT

Aug 29, 2010
22

Sorry, I'm a bit con-fused, (pardon the pun), regarding fusing both input wires.

Would that mean that if there was an overload, that it could be a toss up as to which fuse could blow?

Could that not subsequently lead to someone automatically thinking that the fuse on the live wire had blown, when in fact it was the fuse on the neutral?

Which could in turn lead to them receiving a shock, if they believe that the live wire fuse has blown when it hasn't?

Thanks

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Would that mean that if there was an overload, that it could be a toss up as to which fuse could blow?

In theory it could, however you might be clever enough to place a larger fuse in the neutral line so that overloads would blow the obvious one in the active line. The larger fuse (which would only blow if there was a short between neutral and earth in your device *AND* there was a reversal of neutral and live in your mains feed) would be reserved for weird case failures.

I have never considered placing a fuse in the neutral lead, however in Australia it is not possible to place power plugs into sockets backwards. The most likely cause of A/N reversal is a home-made extension cord. This was a genuine possibility about 20 years ago, but these days a 5 metre Chinese extension cord is cheaper than a single plug.

I think it's pretty obvious, but it is always advisable to treat active and neutral as if they were live (even though one is theoretically at ground potential). Double pole mains switching is always a good idea.

Could that not subsequently lead to someone automatically thinking that the fuse on the live wire had blown, when in fact it was the fuse on the neutral?

Yes

Which could in turn lead to them receiving a shock, if they believe that the live wire fuse has blown when it hasn't?

No. Because they wouldn't open up the device and start playing with it on the assumption that (a) there was a blown fuse, and (b) that they were therefore safe. People who do that are the sort that win Darwin awards.

Before you start to work on mains operated equipment you turn it off, unplug it, and (if possible) remove the power cord. And even then, you assume that there will still be a charged capacitor there waiting to bite you.

If, for some reason you have to operate on mains equipment that is still connected to the mains (even if the mains are switched off, or the equipment is isolated by its power switch), you still assume it is live until you can conclusively prove it's not (and even then you still work cautiously).

#### Militoy

Aug 24, 2010
180
...I have never considered placing a fuse in the neutral lead, however in Australia it is not possible to place power plugs into sockets backwards. The most likely cause of A/N reversal is a home-made extension cord. This was a genuine possibility about 20 years ago, but these days a 5 metre Chinese extension cord is cheaper than a single plug....

In the USA the plugs are polarized as well (wide blade neutral) - but working in a test environment, I've seen more than a few test benches with built-in power strips wired backwards. Also - a real popular item in the lab is a little plastic extension plug, that drops the safety (earth) ground pin. A lot of techs believe that by using one, they are "isolating" their test equipment - and avoiding ground loops. Go figure.

The 2-fuse method would be an exception - more commonly, I might use a 2-pole breaker, or reinforced (double) insulation to chassis. However - I've seen real-world cases when a Line short to chassis would trip the fuse in the (reversed) Neutral side, leaving the chassis at 120V potential to ground. Not a problem - until a grounded human touches the case of the defective equipment.

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