Hi Marius,
Your scenario can happen. I recently saw B- arcing to the chassis through
the insulation on low voltage wiring in an LK500-ZB amplifier. It had
suffered an HV arc during which the grid meter shunt resistor blew open.
This resistor was the only thing holding B- near ground, so it could now
float downwards to a dangerous value. An unwary person trying to probe the
low voltage end of things in this situation could get a nasty surprise.
This is why one of the recent topics on the reflector has been the use of a
stout diode from B- to ground, so that in case of a fault B- can't go far
below ground.
73, Carl WS7L
> -----Original Message-----
> From: amps-bounces@contesting.com
> [mailto:amps-bounces@contesting.com] On Behalf Of Marius Hauki
> Sent: Tuesday, September 14, 2010 4:46 AM
> To: amps@contesting.com
> Subject: [Amps] Possible issue with B- run in wire with less
> than HV rating? (Separate HV PSU and RF decks)
>
> Hi. I have been following the interesting safety discussion.
>
> Let us consider this scenario: we have a separate amp and HV
> deck. The anode has a potential of 3650 volts on it. The cap
> bank in the HV deck is fully charged.
>
> We have these wires between the HV supply and the RF deck:
>
> B+ lead: SHV plugs and receptacles in both ends and RG59 between.
> Chassis ground is connected to the plug shell on both sides.
> B- lead: The B- is a normal relatively heavy gage cable and
> it is connected with a normal plug and receptacle with
> isolation rated to 1kV . The amp is grounded grid, so we run
> a separate B- lead as mentioned. The B- lead is rated to 1kV
> in terms of insulation.
>
> Fault:
> We now for some reason have a short between the anode and the
> grid or the anode and the chassis.
>
> B- becomes minus 3650V compared to the chassis
>
> Since B- has only a 1kV insulation, and now sits at -3650V
> there is a danger of arcs between B- and chassis (now at at
> zero potential). The chassis to B- resistor has failed, for
> some reason and is open circuit.
>
> The operator happens to hold the B- cable for some "cable rearranging"
> behind the rack. It is not unplugged so it should be safe (he
> is nowhere near the B+ cable). He also leans on the the
> chassis where the amp subrack is grounded with a copper strap.
> Now there is a sudden arc from the B- to the operator. The
> current runts from chassis ground thru the operator to the B-
> connector.
>
> Doesn this mean that we should also use HV insulated wire on the B- ?
>
> 73
> LB3HC
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