Fuses rated for DC circuits ?

[Pscammp]

Hello all,

    Im in the process of planning a field battery project to power my kit but have come across an issue I'd like some clarification on if anyone can help.

 

Bare in mind that all our kit works on DC power, I have read that you shouldn't use fuses designed for AC in a DC circuit, some im sure will know the

obvious reasons for this. So, Google searching I went.

 

Would'nt you know it, I am after some DC fuses suitable for 12 - 19.5v max range  either 20mm or 31mm long which are rated for DC between 1A and 6A

fast / quick blow and can I find any here in the UK, actually, any type of DC spec fuse......Like hell can I.

 

Can anyone suggest a supplier here in the UK who can supply genuine DC spec fuses of either 20mm or 31mm long (prefer 31mm)  ????

 

Many Thanks for any help

 

Paul

 

 

[Stub Mandrel]

The only issue you may find is that some fuses will have different voltage ratings for DC or AC use.

I would avoid 'mains fuses' but any off-the shelf 15mm quick-blow fuses from the likes of Maplin or CPC should be fine.

Neil

[michael8554]

Hi Paul

Search for 20mm or 30mm Car Fuses, they are rated for 12-14V and come in many amperages.

Michael

[baldeliftman]

? are you just using one item on the battery or several if so then surely you should fuse each item separately

[Pscammp]

Nebula,

    It will be multiple items and I will be fusing them separately. It will be: 

NEQ6 Pro

QHY8L

12v 7 way USB 3.0 Hub,

Moonlite Focuser controller

150w DC-DC module for Laptop output

(future dew heater controller)

 

Looked at Maplin's quick blow glass tube type fuses but even these are rated as 250VAC

 

Probably will go with the following:

http://www.ebay.co.uk/itm/251990715396?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

 

At least using 'standard blade fuses' they are pretty much designed to be used in a 12DC environment and I can get

fuse ratings to cover all the above very easily. Not quick blow types but there you go

 

Regards

Paul

 

 

 

  

 

[Paul M]

In most AC applications fuses are there to protect cables and installations from prolonged overload or fault protection. Ultimately it's to prevent fire or permanent damage to equipment that is quite robust.

Even the fuses in appliance plugs are only designed to protect the flex from overload. For the appliance to blow that fuse means there is already a terminal defect in the appliance.

So carrying that forward to inline DC equipment fuses, what you are most likely protecting is the power supply and possibly, if it's a  high current supply such as a leisure battery, the cables.

If a piece of electronic equipment blows a correctly rated inline fuse then it's probably already toast! I'm not saying that DC fuses are not a serious consideration but it's worth considering just what the limitations are when chosing them.

Sorry, yet again I've answered a different question to the one being asked! :-)

As already said, Maplin are a good source of such gear. 20mm instrument fuses are the way to go.

[Oddsocks]

Paul,

Car blade fuses are totally unsuitable for your intended use, they react far too slowly and the tolerances are far too wide.

It might help to clear up the confusion over voltage ratings for standard glass or ceramic cartridge fuses which there seems to be a lot of rubbish posted over the internet.

There is no difference in construction between a fuse rated for AC or DC use except for when very high voltages or high frequencies are used.

For example, in the case of a normal a glass/ceramic 20mm x 5mm body fuse rated at 250vac (50hz) and 4A, it is generally accepted for designers that in a DC circuit the voltage is derated by 50% so it can be used at up to 125v dc and all the fuses inherent characteristics remain the same.

You could use this fuse with a supply voltage of 1v dc or 125v dc and it will still blow when a current of 4A flows acoss it and take the same ammount of time to do so.

The reason there are so few fuses that actually carry both an AC and DC rating is because it is unnecessary and would require the manufacturers to undertake extra certification and testing that would make their fuses too expensive and uncompetitive.

So for your intended application you can take any standard glass/ceramic 20mm or 30mm fuse rated at 250vac slow, normal, fast, super fast or anti-surge and use it at any DC voltage at up to 125vdc and it will perform in every way to its full 250vac rating.

Fot those reading this and wondering why a 250vac fuse is derated to half that voltage,125v when used with dc it is to do with the way fuses fail and has nothing to do with insulation. When the fuses rated current is exceeded in a fuse on an ac supply, the current begins to exceed the allowed nominal as the ac waveform climbs to its peak and the fuse begins to melt, the fuse wire parts and a gas of ionized metal particles forms between the melted ends allowing the current to continue to flow, as the ac waveform falls back to the zero crossing then the ionised flow collapses and the gas disperses, the fuse wires sag and fall away from each other and on the next rise of ac voltage maximum the wires are too far apart to sustain an ionized flow, the fuse has fully blown so the maximum response time for this type of fuse is equal to half the ac period plus the melt time of the metal. When this fuse is used on a DC circuit there is no zero crossing or reversal of flow and if used with the full 250v DC then the ionized gas will persist between the melted ends untill much more of the fuse wire has melted, the reaction time under these circumstances will be much longer, perhaps ten times or more, this depends on many variables such as ambient temp, air humidity and pressure, whether the fuse is air filled, gas filled and sealed or silica filled. When used at the derated maximum of 125vdc a 250vac fuse will not have sufficient voltage across it to sustain an ionized flow and its reaction time will be within the tolerances quoted for 250vac operation. 

It is also important to know why you are installing a fuse in the first place, a fuse primarily is to protect against damage to the supply network and against fire, it offers no electrical or electronic protection to the components it is supplying, the fuse will only blow if something has already failed and is damaged in the equipment, or if the fuse was rated too low in the first place and has become fatigued. In the case of a component failure the fuse blows to prevent futher damage by fire to either the supplied equipment or to the supply network be that a battery, the household wireing or the power station and all the cabling inbetween.

Only once very high DC voltages or high frequency AC currents do very speciallised fuses have to be used and for your use normal 250vAC 20mm or 30mm fuses will perfectly adequate, if you want to be extra cautious then use anti-surge fuses, these contain a small spring pulling the fuse wire taught and when the fuse wire melts the spring pulls the wire ends apart preventing an ionised field sustaining a flow, the response time of these fuses is the same for both AC and DC as it does not require a wait until the next zero crossing time for the current flow to stop. When using anti-surge fuses you have to be aware that the inrush current at swich on can be many times that of normal running so anti-surge fuses are normally rated for a higher current than a normal fuse.

I still have a suitcase full of fuses from my pre-retirement days and these were for use on AC and DC voltages from millivolts up to twenty killovolts and the ones rated at 250vAC were fitted by the designers in low voltage DC circuits of highly complex medical electronics used for life support systems, these systems are certified to much higher standands than those required by domestic electronics and there were no special DC rated fuses used in those!

Stop beating yourself up unnecessarily and use standard 20mm or 30mm 250vAC fuses and these will be fine for DC up to 125v.

William.

 

[Stub Mandrel]

The difference is that the arc produced when a DC fuse blows is not self-extinguishing (an AC spark goes out when the voltage crosses zero). You don't really need to worry about this effect at anywhere up to a 24V system as the gap left when fuse blows will be too long to sustain an arc at these lower voltages.

 

My understanding of anti-surge (t-rated/time delay) fuses is that the spring absorbs the thermal shock allowing the fuse to briefly pass a surge current (e.g. at switch on) without blowing.

Fuses for really high currents/voltages (especially DC) are often filled with sand which 'mechanically' extinguishes the arc.

[markthg]

If you want fast blow then you want to avoid car fuses.

Try Maplins if not RS Components.

[michael8554]

Car fuses are just like any other fuses, they come in slow and quick blow varieties.

12 - 19V quick blow fuses, 30mm long, in a variety of amperages, what's the problem?

Michael

 

[Oddsocks]

2 hours ago, michael8554 said:

Car fuses are just like any other fuses, they come in slow and quick blow varieties.

12 - 19V quick blow fuses, 30mm long, in a variety of amperages, what's the problem?

Michael

 

The type of "car fuse" that both myself and Markthg are saying are not suitable are the microblade ATO, ATC and ATF type that are found in most modern car fuse boxes, plastic bodied with the fuse element and blades punched from a single piece of sheet material, the manufacturers specification documents quote very wide tolerances, for example the 6a 32v ATOF microblade made by the Little Fuse company quotes a tolerance on opening time of between 250 milliseconds and 50 seconds @160% overload. While this might be quite acceptable for a car, lighting circuit or windscreen wiper motor, 50 seconds is a very long time if you want to protect a laptop 12v to 19v step up converter from bursting into flames?

You do wonder somtimes seeing the number of burnt out new cars at the roadside with wiring loom failures if the specification for car fuse boxes is a bit overdue for an upgrade.

The standard 12Vdc-19Vdc, 30mm long, glass/ceramic bodied quick-blow fuses made with cold drawn fuse-wire you refer to, and available from reputable car spares retailers are quite adequate (provided they have some kind of industry certification and are not cheap unregulated imports),  just avoid the "blade" type fuses for use with electronic devices.