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Re: Need fuses for Multimeter

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On 1/31/2018 5:50 PM, Ralph Mowery wrote:
> In article <2i647dla33h4gegvdpqbqn6k5ugu9qeein@4ax.com>,
> oldschool@tubes.com says...
>> Fuses that have a higher voltage rating are fine to use in the low
>>> voltage applications.  They only work on the curent.  In those
>>> inexpensive meters with fuses only rated for 250 volts, they could be
>>> dangerous at higher voltages.  We were shown a trainging film  by the
>>> Fluke salesmen.  They showed what could hapen  if you had a low voltage
>>> fuse and had the meter set for ohms or amps and put it across a 480 volt
>>> line with plenty of curent.  The fuse would arc over and the leads would
>>> melt down.  You would too if you were holding the leads.  They make some
>>> special fuses for them to get the higher CAT ratings.
>>>
>>
>> Ok, that makes sense. Apparently the element inside the fuse must melt
>> for a wider gap on higher voltage rated fuses. Which would occur because
>> of the type of metal used and/or shape of the wire. I have seen fuses
>> with that element looking wavy, so I suppose that has something to do
>> with it. This is all mostly just a guess, but something has to make that
>> gap wider to prevent arc-over.
>>
>> I can see arcing at voltages above 250 V. I have worked on lots of old
>> tube tv sets, and have seen the B+ high voltage jump across tube socket
>> pins and other places. Most of the time the voltages were from 300 volts
>> up to nearing 600 volts. Apparently these meters are not really suited
>> for that higher voltage tube gear, even though both the AC and the DC
>> ranges go to 1000 V.
>>
>> I do find the 32 V limit on some of those fuses sort of an oddball
>> figure though. Any voltage from 32 to 100 V is not going to arc much.
>> Even 120 V AC line voltage really does not arc very much. Most of the
>> arcing I have seen has been over 250 V. The CRT anodes on old CRT tv
>> sets were notorious for arcing, but that was several thousand volts. I
>> have seen (and felt) the arcing from electric livestock fences (that
>> bites, and always makes me use my worst vocabulary). Those fences are
>> generally 2000 to 6000 volts.
>>
>> Anyhow, this has been an interesting thread. I learned more about fuses,
>> which I have used for decades but never given much thought to them,
>> other than selecting one that is the right amperage and fits the holder.
>> It is likely I have used automotive fuses on tube gear that were not
>> rated for the voltage. Now I will be more aware of that.
>>
>> Speaking of arcing. There is a youtube video where a main high tension
>> line on a tower is disconnected, and that forms am arc that keeps
>> growing taller. (Called a Jacobs Ladder). If I recall, the voltage is
>> something like 500,000 volts. The guys open 3 switches on the tower (3
>> phase), and that arc occurs. Those switches are probably one of more
>> feet of gap, but at that voltage it dont seem to matter. That's an
>> awesome video.....
>>
>>
>>
> 
> There is a lot more to the fuses than meets the eye..
> 
> I don't know about the low voltage fuses, but wild guess for the 32
> volts is most of them were used in cars and trucks where 24 volt
> batteries were the highest and the 32 was some safety factor.
> 
> The 250 volt is good for common house voltages in the US.
> 
> My Fluke meter has 2 fuses in it and they are both rated for 1000 volts.
> One is for 11 amps and I don't recall the lower curent one.  That is so
> it will meet the CAT 3 requirements.  If you have not heard of the CAT
> requirements, you should look it up on the internet.

The fuses are not just voltage rated. The fuses Fluke uses are rated for 
interrupting on circuits with high available fault current.

The available fault current rating is very familiar to electricians 
working on power circuitry, like where you used to work.

"Category rated" meters need to be used on high capacity power equipment 
- you must have used them where you used to work.

Why use the right fuse, and in particular Cat rated meters:
https://www.youtube.com/watch?v=Hp1JdVwbN_U
and
https://www.youtube.com/watch?v=P35HRYHFz7c

"Arc flash" has been recognized as a danger, and provisions are creeping 
into the US National Electrical Code, and it is a particular issue with 
OSHA. "Personal Protective Equipment" may have to be worn when access 
covers are removed, and some equipment is not safe to work on live.

The smartest electrician I have met was on a trouble call to a food 
plant. He wanted to measure the motor current on a moderately large 
motor. The only place to do that (and a good location) was the motor 
starter. It was in a 480V motor control center - a frame with module 
spaces that motor starter "buckets" were installed into. He opened the 
door and put a clamp on ammeter on the motor lead, absolutely common 
practice. It is not known what happened next, but there was an arc 
flash. He was in the hospital for quite a while. Some of his injuries 
were from copper vapor condensing on his skin. He looked younger after 
the plastic surgery. Among the damage to the plant - one of the high 
voltage primary fuses to the supply transformer for the building blew 
and its fuseholder was damaged and had to be replaced.

It can be a problem at lower voltages too. A downtown building had a 
mere 208/120V service, but high available fault current with 4 or 6 
supply conduits. The service 'burned down' (no one around). Some of the 
service wires burned back into the conduits, with a few welding 
themselves to the conduit. (The were attached to the utility transformer 
vault bus bars with "cable limiters", a combination lug and fuse - fuse 
was blown.) Some of the wires burned back and remained live.

Some of you probably have exposure to high capacity power equipment, 
like where you used to work.

Phil covered some of this also.


> I don't know wht
> the voltage ratings are for my old Simpson 260, but should look one day.
> I do have  abox of fuses that will fit in my Fluke but they are only
> rated for 600 volts AC.  As I don't work around anything but around the
> house I would use them if one of the origional ones blow.  If I was
> still working and around the 480 vot 3 phase stuff that had 500 amp or
> more fuses in it, I would only replace with the origional type.
> 
> I do have a hand full of the Harbor Freight 'free' voms scattered around
> the house and cars.  Good enough for quick checks.  They are really very
> accurate for the price (usually less than 1 % off the Fluke) as I
> compaired them with some of the Fluke meters.
> 
> I do remember drawing arcs of several inches off the old black and white
> TVs.  I think they ran about 10 to 12 thousand volts.
> 
> You mentioned the Jachobs ladder.  I have made them out of old neon sign
> transformers and some furnace igniter transformers and some rods about 2
> feet tall.
> 
> 

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Thread

Need fuses for Multimeter oldschool@tubes.com - 2018-01-30 14:56 -0600
  Re: Need fuses for Multimeter "Tom Del Rosso" <fizzbintuesday@that-google-mail-domain.com> - 2018-01-30 17:15 -0500
    Re: Need fuses for Multimeter Ralph Mowery <rmowery28146@earthlink.net> - 2018-01-30 18:17 -0500
    Re: Need fuses for Multimeter oldschool@tubes.com - 2018-01-31 14:30 -0600
      Re: Need fuses for Multimeter dplatt@coop.radagast.org (Dave Platt) - 2018-01-31 14:53 -0800
      Re: Need fuses for Multimeter "Tom Del Rosso" <fizzbintuesday@that-google-mail-domain.com> - 2018-01-31 18:48 -0500
  Re: Need fuses for Multimeter "Tom Del Rosso" <fizzbintuesday@that-google-mail-domain.com> - 2018-01-30 17:55 -0500
  Re: Need fuses for Multimeter Ralph Mowery <rmowery28146@earthlink.net> - 2018-01-30 18:14 -0500
    Re: Need fuses for Multimeter oldschool@tubes.com - 2018-01-30 18:58 -0600
      Re: Need fuses for Multimeter Ralph Mowery <rmowery28146@earthlink.net> - 2018-01-30 22:38 -0500
        Re: Need fuses for Multimeter oldschool@tubes.com - 2018-01-31 03:00 -0600
          Re: Need fuses for Multimeter Ralph Mowery <rmowery28146@earthlink.net> - 2018-01-31 10:06 -0500
            Re: Need fuses for Multimeter oldschool@tubes.com - 2018-01-31 14:07 -0600
              Re: Need fuses for Multimeter Ralph Mowery <rmowery28146@earthlink.net> - 2018-01-31 18:50 -0500
                Re: Need fuses for Multimeter bud-- <null@void.com> - 2018-02-01 22:25 -0600
                Re: Need fuses for Multimeter Ralph Mowery <rmowery28146@earthlink.net> - 2018-02-02 14:15 -0500
    Re: Need fuses for Multimeter oldschool@tubes.com - 2018-01-31 14:23 -0600
      Re: Need fuses for Multimeter Ralph Mowery <rmowery28146@earthlink.net> - 2018-01-31 18:11 -0500
        Re: Need fuses for Multimeter oldschool@tubes.com - 2018-02-02 15:07 -0600
  Re: Need fuses for Multimeter M Philbrook <jamie_ka1lpa@charter.net> - 2018-01-30 18:18 -0500
  Re: Need fuses for Multimeter Brian Gregory <void-invalid-dead-dontuse@gmail.com> - 2018-03-20 04:33 +0000
    Re: Need fuses for Multimeter Ralph Mowery <rmowery28146@earthlink.net> - 2018-03-20 09:58 -0400
      Re: Need fuses for Multimeter Brian Gregory <void-invalid-dead-dontuse@gmail.com> - 2018-03-26 23:20 +0100
  Re: Need fuses for Multimeter Badbob <badbob45@wi.rr.com> - 2018-05-25 10:22 -0500

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