Fuses in both +ve and -ve supply rails? - best practice?

[Al (astral highway)]

I'm rebuilding the entire power supply of a piece of military test equipment from 1942.

While I'm at it, I will introduce fuses to the set, which has an isolating transformer. I note that a number of circuits I've seen recently have fuses in both +ve and -ve supply rails -- why and when is this a good idea?

Also, where should I draw the line? This isn't just a safety thing - I've had to replace the transformer and a few other major components, so it's not going to be a cheap restoration. I want to make sure I protect the finished article!

mains fuse, fuse in live feed to xformer primary, fuse after voltage regulator in rectified HT+, fuse (slow blow) in heater chain?

what do others do?

thanks

[HG MICKE]

I would suggest a Fuse between Transformer and Rectifier,protecting secondary in the event of failure of smoothing / reservoir capacitor or rectifier.

Colin

[Ed_Dinning]

I think the rule should be that if there is a short then a fuse should blow, so if you have a 350-0-350 transformer, for complete protection you need a mains input fuse and one in each 350 leg. Typically ther is normally only one in the 0 leg as there is not much loikely hood of a short between anodes in most rectifiers.
However in the Mullard valve tester there are several windings on the HT transformer ; 450, 350, 250 -0-250,350,450 or something similar. With a faulty card it would be possible to short the 450 on one side to the 250 on the other, not enough current may flow to blow the primary fuse so it would be prudent to fuse all HT line including the 0.

Ed

[Skywave]

And this Thread has prompted me to ask:

Why - on some vintage kit - even when a mains-isolating transformer and a 3-core mains lead is fitted with the chassis is connected to supply "E" - are there chassis-mounted fuses on the mains input "L" and "N" leads?

Could it simply be that that was an extra safety 'precaution' - just in case someone wired the 5-amp, 3-pin (round pins) plug wrong ("L" to "N" supply and vice-versa) ?

One of those many little things that has always puzzled me ; but then again, sometimes I have a difficulty in seeing the glaringly obvious

Al / Skywave.

[Al (astral highway)]

Originally Posted by Ed_Dinning

I think the rule should be that if there is a short then a fuse should blow, so if you have a 350-0-350 transformer, for complete protection you need a mains input fuse and one in each 350 leg. Typically ther is normally only one in the 0 leg as there is not much loikely hood of a short between anodes in most rectifiers.
However in the Mullard valve tester there are several windings on the HT transformer ; 450, 350, 250 -0-250,350,450 or something similar. With a faulty card it would be possible to short the 450 on one side to the 250 on the other, not enough current may flow to blow the primary fuse so it would be prudent to fuse all HT line including the 0.

Ed

Good call. I'll follow that advice.

[plumbweiss]

Certainly on-board fusing of the L and N is still common in equipment designed for single phase use in countries where it's the norm for reversible mains plugs (eg shucko). That way, a live to earth fault will always blow a fuse no matter which way round you're plugged in.

[Al (astral highway)]

Originally Posted by plumbweiss

Certainly on-board fusing of the L and N is still common in equipment designed for single phase use in countries where it's the norm for reversible mains plugs (eg shucko). That way, a live to earth fault will always blow a fuse no matter which way round you're plugged in.

Ah, that explains it. I hadn't thought about the reversed L-N problem. Thanks.

[peter_scott]

Originally Posted by Skywave

Why - on some vintage kit - even when a mains-isolating transformer and a 3-core mains lead is fitted with the chassis is connected to supply "E" - are there chassis-mounted fuses on the mains input "L" and "N" leads?

Al / Skywave.

Hi Al,

It might be because in some countries there is no neutral. In Spain for example you will find ground potential mid-way between the two incoming supply connectors. I presume the philosophy is that most electric shocks are from conductor to ground rather than conductor to conductor and thus you get fewer fatalities.

Peter

[pmmunro]

The title asks about fuses in positive and negative rails, implying DC supplies since the terminology is not appropriate for AC supplies. However, most of the discussion seems to be about AC circuits.

Perhaps because everyone is well aware of it, no-one has made the point that fuses should never be fitted in the neutral of supplies which have the neutral earthed as this could lead to the whole of the circuit remaining at a dangerous potential, although apparently disconnected, should it be the neutral fuse which blows. The same principle applies to single-pole switches and circuit breakers. Two pole switches must always be mechanically coupled so that there is no danger of disconnected only the neutral.

This is very clearly stated in all electrical safety regulations including BS7671, the IEE wiring regulations.

Sorry if this point is well understood, but the correspondence on the forum suggests that experience varies widely, and this is important.

In any case there is little point in having more than one fuse as a fault to earth near the neutral end of the circuit is much less likely to blow the neutral fuse and a simple overcurrent will flow in the entire circuit, blowing a fuse wherever it is fitted, so that a single fuse in the phase side gives adequate protection.

PMM.

[Al (astral highway)]

Hi PMM,

Thanks for posting that.

I was, as you point out, more focussed on best practice for DC circuits than anything else, but I guess I may have unclarified that by going on to talk about 'heater chains.'

But I did find your point about neutral fusing of neutral earthed supplies interesting. This is clearly a possible error with transformerless mains equipment that comes from a car-boot sale / online auctions/ and may have reversed plug connections, so worth watching out for!

[plumbweiss]

no-one has made the point that fuses should never be fitted in the neutral of supplies which have the neutral earthed

I think this is actually permitted under CE.

The issue is that if there is only one fuse, it should be in the live. A fuse in both is OK. If the neutral fuse fails due to overcurrent, the appliance ceases to draw current and is rendered safe, if it's a live to earth fault then the live fuse blows and makes it safe. Of course the neutral fuse wouldnt blow with an earth fault, at least not if connected in the UK via a BS1363 system....which always has the plug-top fuse in the live.

We supply CE approved 230VAC electrical equipment to the UK from a reputable German manufacturer and they always come with both L and N fused. Internal fuses, after all, are generally to protect the appliance against overcurrent/fire etc.

Regarding the DC rails, I would fit a fuse in both legs if it's a split symmetrical supply for a class-B amp, for example and the PSU couldn't be protected in any other way.