Circuit breakers and fuses come in various types with various
characteristics. For example, the average molded case residential circuit
breaker may take c. 30 to 50 seconds to trip, at 300% rated current. Higher
currents cause quicker tripping, and a point is reached where there is
instantaneous tripping. This point is usually tested at 10x the rated
current. Some breakers don't trip at all...right out of the box.
Fuses can provide much faster protection. Current limiting fuses open and
clear in less than 1/2 cycle but are usually quite expensive. I've applied
both across a short circuit and it is amazing how quickly the current
limiting fuse clears the fault, with obviously less disturbance to the
circuit.
It is worth looking into the parameters of fuses. Though current limiting
fuses are expensive, I recall lower priced fuses that provide a degree of
current limiting.
Though I'm far from a protection expert, I have made a lot of intentional
"big bangs" and observed the results.
Jeff - K1LE - CT ><>
----- Original Message -----
From: "Ian White, G3SEK" <G3SEK@ifwtech.co.uk>
To: <amps@contesting.com>
Sent: Monday, November 17, 2003 2:43 AM
Subject: Re: [Amps] L-7 Failure, HV fuses
> R. Measures wrote:
> >>Re HV fuses:
> >>
> >>I have seen in the literature descriptions of a HV fuse made by
inserting
> >>into the B+ line, a 30 guage Cu wire approx 2" long. suspended
between
> >>two ceramic standoffs in the amp. Would that function as a near
> >>instantaneous HV fuse. protecting the tube in the event of a heavy
current
> >>HV fault?
> >
> >** 2" of #30 Cu has about 0.017-ohms of R. Since the ESR of
> >high-ripple I 450V electrolytics is c. 0.25-ohms each, in a typical
> >3200VDC anode supply, the peak discharge I would be 3200V / [(8 x
> >0.25-ohms) + 0.017-ohms] = c. 1600A-peak. IMO, this does not provide
> >adequate protection. As I see it, HV fusing is not needed if the
> >primary of the HV transformer is fused or circuit-breakered. What is
> >needed is a device to limit peak discharge-I -- i.e., a sturdy-enough
> >glitch-R; typical values of which are 10 to 30 ohms.
> >
>
> Totally agreed - the difference between a resistor and a fuse is that
> the resistor provides instantaneous current limiting. A fuse does not.
>
> That's why the glitch resistor is the key component. You need a fuse as
> well, but that can be a mains fuse in the transformer primary.
>
> A "sturdy enough" has to be capable of dissipating tens of watts in
> normal operation, and also capable of withstanding the full HV from end
> to end if a major current surge occurs. In practice this means quite a
> large, long-bodied resistor.
>
> Unfortunately a resistor of this size may be difficult to fit into an
> amp that doesn't already have one - but do it, any way you can.
>
>
> --
> 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
> Editor, 'The VHF/UHF DX Book'
> http://www.ifwtech.co.uk/g3sek
> _______________________________________________
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> http://lists.contesting.com/mailman/listinfo/amps
>
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