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#137830 - 07/31/03 11:32 AM Protection systems
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
In our Regulations over here, it states that for poly-phase equipment, a system of protection must be used, that in the occurence of a fault on one phase of the equipment, the other phase or 2 phases, must also be disconnected from the supply when the first phase trips.
That's OK if you have a linked MCB on the circuit for each phase, but what if the protection is HRC, with individual fuse-links for each phase.
My question is, do you have this sort of Regulation where you come from?.
And if so, how is compliance normally achieved?.
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#137831 - 07/31/03 12:01 PM Re: Protection systems
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Oh, this is one of my pet peeves:

This requirement is in the Swedish regulations, but then the authority issues a statement: "Ignore it. It's not the way we do things here in Sweden." A blown main fuse is common source of problems and people have difficult to find it.

This isn't the only case where the authority asks you to ignore its own rules: Same goes for mains bonding and earth wire to street lights. The latter requirement was dropped since that is not the way things are done. New regulations will be issued next year. These will only consist of fifty pages and will not contain anything technical at all. Pretty hard to violate those rules...

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#137832 - 07/31/03 04:16 PM Re: Protection systems
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Yep, HRC fuses don't comply with the "one-phase-out, all-out" approach and can result in motors single-phasing.
But there are places where the prospective short-circuit current in a circuit exceeds the rated breaking capacity of an MCB, so it has to be fuses.

I'm not involved with 3-ph very much, but surely some systems use a monitor circuit linked to a contactor?

If a fuse blows the monitor detects the loss of a phase and opens the contactor, shutting off power completely.

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#137833 - 08/01/03 07:46 PM Re: Protection systems
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
C-H,
That's right, you guys use 3 phase in houses in Sweden?.
 Quote:
"Ignore it. It's not the way we do things here in Sweden."

That's a bit of a strange about-face.
Paul,
HRC fuses are used quite prolifically over here in Industrial and Commercial installations, mainly because when most of these places were wired(mid-late 60's), the only type of boards available, had HRC pan units in them.
Like you said, this is really only a problem where motors are concerned, but judging by the number of burned-up motors I've replaced over the years, this is quite a common occurence.
Yes, a Phase Failure Relay is normally used to monitor the current in the feed wires and if this gets out of hand, it opens a contactor control circuit.
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#137834 - 08/16/03 03:41 AM Re: Protection systems
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Paul,
That's very true about the PSCC being higher than the Rupturing Capacity of a protection device, we have MCB's of up to 10kA and above that 80+kA HRC fuse links are used.
With PSCC's of 5kA+ here, especially where switchboard panels are located close to transformers and heavy lines, the PSCC can be quite phenomenal.
I did a Fault current test on an Industrial installation last week and I came up with a PSCC level of 35kA at the incoming terminals, short one of these phases out to the edge of the panel with an Un-insulated screwdriver and see the sparks!.
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#137835 - 08/16/03 08:21 AM Re: Protection systems
djk Offline
Member

Registered: 12/26/02
Posts: 1269
Loc: Ireland
Once again our regs go a little over the top on this issue in Ireland.

Small 3 phase distribution panel would have:

Supply side fuses, (more modern installations will use interlinked breakers too) feeding through the meter and an isolating switch (usually a turn handle type)... then on to the distribution panel .. (sealed ot this point by the PoCo)

Bonding to earth etc is also done by the PoCo and sealed (same with 1phase domestic stuff).

3 X Neozed fuses
followed by an one out all out circuit breaker which must be lower rated than the fuses (which are there incase of everything failing or getting stuck)

and then each circuit has one out all out circuit breakers

Anything connected to a socket of any type up to 32Amps must also be RCD protected. (Same goes for any lighting circuits or 13amp sockets taking a single phase +N)

Some fixed equipment that would have problems with RCDs would be exempt but other than that the rule applies to everyting up to 400V.


-----

The Neozed fuses on our commercial/domestic distribution panels are supposed to be there primarily as a fire prevention measure. Basically if the panel ever caught fire they're not electromechanical and will blow even if there is severe melting going on...I know this sounds quite over the top, but it's the logic behind keeping fuses there.

[This message has been edited by djk (edited 08-16-2003).]

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#137836 - 08/17/03 04:34 AM Re: Protection systems
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
The breaking capacity issue was raised in the NEC forum a few days ago.

It's quite common here to have 6000A-rated MCBs on a domestic panel even though the prospective short-circuit current might be much greater. The PoCo's main HRC cartridge fuse is considered to be adequate protection against a drastic bolted short in which the breaker fails to open.

The old rewireable (BS3036) fuses are rated at only 2kA breaking capacity!

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