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#27169 - 07/03/03 05:30 PM cartridge fuses
circuit man Offline
Member

Registered: 11/05/01
Posts: 287
Loc: saluda,s.c.
here's one for a lively disscussion guys. was called to fix a hot water heater today standard old 240/4000 watt unit.found that it needed a new upper thermostat due to water getting in.checked the power that is connected to a small safety switch with fuses.person that installed it last had two 30 amp fuses on a #12 wire.no ground on the heater case at all. so go & replace the fuses & wire with the correct size.turn on the switch & its pullling 28 amps through a 20 amp fuse.so i cut it off & discover picked up wrong thermostat.go back & put in correctone & load drops to 14 amps(one element). the funny thing is the fuses should have blown almost immediately from overload.when i took them out after a min or 2, you couldn't even hold them.took them back to the hardware store & told the manger, he might have a big problem.he first asked if my amp meter had been calibrated, told him nope i trusted my fluke meter, not to mention the wire was getting hot. btw these were not buss orothr fuses but a wel known hardware chain ---. what do you guys think? i belive hey were miss labeled, that means DANGER!

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#27170 - 07/03/03 05:55 PM Re: cartridge fuses
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Different types of fuses have different characteristics, but in general I wouldn't expect a 20A fuse to blow on a 28A load after only one or two minutes.

Many fuses will actually carry a 50% overload for a lot longer than is commonly believed.

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#27171 - 07/03/03 06:10 PM Re: cartridge fuses
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
20-amp NON-type fuses will open in roughly 5 minutes at a 30-amp {150%} load. “Fuse curves” are generally referred to as inverse-time-current characteristics. See if the suspect fuse has it’s rating also stamped on the ferrules.

Not sure if it’s always the case, but {before “deregulation”} one used to be able to take a personal meter into the local-utility metering cal lab and if not busy they would compare your meter to their “bench standards” without any written report. A local met lab charges $45 to “sticker” a Fluke handheld, without any “as found” documentation. [If you have an accessory current probe, that costs extra.]

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#27172 - 07/03/03 09:02 PM Re: cartridge fuses
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
circuit-man,
What sort(Category of Rating) was the fuses that you had?.
To my way of thinking, a P2 type fuse will rupture at 1.5 times the current rating of the fuse, ie: 20A x 1.5 = 30A.
A R-rated fuse will operate at 2 times the current rating of the fuselink, ie: 20A x 2= 40A.
Please note that all fusing is on (or should be) an inverse time-current curve(ie: more current, less time).
Also I find it hard to believe, that a said element would draw more current with a different thermostat connected.
I do heaps of Faults callouts(mostly at night, to people that have no Hot Water)
and 9 times out of 10, it is the Element that has failed, although, I have seen heaps of Thermostats fail.
A lot of people that I have seen, cannot get thier heads around the idea that it takes about 6-7 hours to heat up a HW Cylinder, they think that it is just like boiling the jug, for a cup of tea!.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#27173 - 07/03/03 11:05 PM Re: cartridge fuses
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Typical US water heaters have two elements, upper and lower; each 1.5-5.5kW, with a double-throw thermostat that select one or the other. What may have happened was both elements were energized simultaneously, which generally is not intended.

Example see page 1 “non-simultaneous operation” http://www.hotwaterproducts.com/BWC/Parts/Pdfs/Electric%20Wiring.pdf

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#27174 - 07/04/03 02:01 AM Re: cartridge fuses
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Bjarney,
Thanks for your input on this one!.
How do the fuse ratings in the US, compare
with this:
  • P class fuses- Operate at a Fusing Factor of 1-1.25 x Rupture Current(Normally Protects Semi-Conductor Installations)
  • Q1 class fuses-1.25-1.5 x Rupture Current(Normally Protects General Lighting and Power Circuits).
  • Q2 class fuses-1.5-1.75 x Rupture Current
    (Normally protects Sub-Mains and Mains in General)
  • R class fuses-1.75-2.5 x Rupture Current
    (these protect Motors and Starters)
  • Special R-class fuses for HV and EHV purposes, these take into account of the High in-rush currents, that occur when replacing one of the 3-phase Delta (mesh) line fuses, normally these are 2.5-3 times Rupturing Current.
    Hence,the big flash when they BLOW!!
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#27175 - 07/04/03 08:25 AM Re: cartridge fuses
circuit man Offline
Member

Registered: 11/05/01
Posts: 287
Loc: saluda,s.c.
thanks guys for the info! i was worried on this because edison base fuses usually blow on a couple of amps overload.now i know the difference.trumpy these were the non type.bj,these had no rating stamped on the end caps what so ever.now guess i know the difference.sorry abut my spelling, it was late & i was tired.

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#27176 - 07/04/03 06:39 PM Re: cartridge fuses
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
North-American fuses differ greatly above and below 600V. The short story is—for lower voltage, there are “one time” {faster acting} and “dual-element” fuses intended for inductive loads like motors and transformers. Above 600V, there are boric-acid/expulsion fuses, versus fully-enclosed current-limiting/silver-sand fuses. There are many tradeoffs in applying fuses, where dual-element and current-limiting fuses being more expensive, but the only practical solution for many cases.

[Of course, fuses and circuit beakers compete, with fuses usually topping out at 169kV and circuit breakers going up to 800kV, but none UL labeled(!), if you can imagine that.]

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