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Author Topic: One fuse or Two?  (Read 6723 times)
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David, K3TUE
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« on: April 27, 2006, 11:45:10 PM »

I am designing a T/R sw which will be triggerable with selectable either 110V AC (unpolarized plug) or 12V DC.  The case of the project will only be ground for RF (not for the mains gnd nor common with the 12V DC gnd).

In an effort to be safe I was planning on fusing the 110 VAC, but I would like to know, since it is unpolarized, should I fuse both lines of it?  I have read elsewhere that one should never double-fuse mains AC, but I think that was in reference to polarized mains plugs with the chassis grounded.
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David, K3TUE
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Don
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« Reply #1 on: April 28, 2006, 02:12:13 PM »

The old a.c. line plugs with the built-in fuses have a fuse in each side of the line.  I recall that the Johnson Ranger uses one of those.

The danger is that the fuse in the neutral side could blow and leave the whole rig at line voltage potential.  But the older non-polarised plugs could be inserted into the outlet the wrong way and achieve  the same effect.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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David, K3TUE
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« Reply #2 on: April 28, 2006, 02:19:02 PM »

Quote
The danger is that the fuse in the neutral side could blow and leave the whole rig at line voltage potential.  But the older non-polarised plugs could be inserted into the outlet the wrong way and achieve  the same effect.

So, in the end, it would be smarter to use at least a 2-prong polarized plug, and simply fuse the hot side.  I've taken the time to check the polarity of all of the outlets in my house anyway for this safety reason, no reason to start doing a half-assed job now.
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David, K3TUE
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« Reply #3 on: April 28, 2006, 02:23:35 PM »

hi Dave,
            I dont possibly see what harm it be if you fused both sides of the incoming AC. With a non polarized plug it would be a good insurance policy. If you go back to the days of the boat anchor transmitters, most had fuses on both sides of the incoming ac. Many even had double fuses right inside line plugs. Just about every one of my transmitters (that I can remember) have double fuses.

To meet current electrical codes you only have to fuse the hot side of the main and not the neutral as the neutral is bonded to ground somewhere in the buildings electrical system. (usually at the fuse / breaker panel).

I guess if you fused the neutral at the appliance, you could possibly run into some strange shock hazard if you had a short between hot and neutral and only the fuse on the neutral opened. But I dont think it would be very likely.

If you are going to have a "floating" load and a non polarized plug a double fuse sounds like the thing to do, just make sure both fuses are of equal value/
      
                                                                   the Slab Bacon
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« Reply #4 on: April 28, 2006, 06:42:47 PM »

Like Don said, 
----  if you have a open neutral (fused blown)l, you may still get zaped...you are still conected to the pole.. a "medium" available path to ground might be, say, sinking 10 amps @ 120V-- 1200W .. not enough to blow the fuse, but it will be  'nuf to burn down the house...might be you.. why take a chance on your house, or your life over a $6.00 safety feature... also, see if your homeowners insurance will pay off for creative cable running...     Can you say "Contributory negligence?"                           


NEC question   Under what situation may a neutral be fused?        Hint: gojuice                Ans. it used to be when the
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RF in the shack


« Reply #5 on: April 28, 2006, 11:09:33 PM »

There was recently a long thread on the Johnson reflector about fusing the neutral.   I think I can safely say that the vast majority of people in the know say never do this.  They also say it's against code so if you electrocute yourself or someone else because of this don't expect your insurance to pay.

Jon
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« Reply #6 on: April 30, 2006, 11:36:30 AM »

Are we talking about fusing the neutral leg in the piece of equipment or in the buildings wiring? Never put a fuse in the neutral leg in the buildings wiring. However, I still dont see a problem with fusing it INSIDE THE TRANSMITTER. Remember that a properly installed transmitter should have several safety paths to ground. The outside shield of your coass cable makes for one, you should have a station common ground for RFI pourposes, that should provide a second, and if you have a grounded plug on your rig, that should provide a third.

Remember you can never have to many grounds on a transmitter, this does wonders to help relieve "rf in the shack" and "ground loop" problems.

Keeping this in mind, if inside the rig you have a short between the phaze (hot) and the neutral, you should blow either of the 2 fuses, If you have a short between the phaze and ground, you will take out the hot fuse. In either scenario. you should not have a shock hazard if everything is grounded properly. Many operators are sloppy about their grounding practices, and that is where you can run into serious problems.

Also keep in mind that a safety (DC) ground is not always an RF ground, but that is another topic for another discussion.
                                                               The Slab Bacon
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David, K3TUE
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« Reply #7 on: April 30, 2006, 01:03:25 PM »

Quote
Are we talking about fusing the neutral leg in the piece of equipment or in the buildings wiring?

the neutral leg of a piece of equipment.

Quote
Never put a fuse in the neutral leg in the buildings wiring.

I never would.
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David, K3TUE
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