[Amps] Possible issue with B- "floating down"

[Gary Patterson]

Could someone comment on the "floating down"  of the b-.  What is actually causing this?

> From: carlclawson at verizon.net
> To: amps at contesting.com
> Date: Tue, 14 Sep 2010 10:28:40 -0700
> Subject: Re: [Amps] Possible issue with B- run in wire with less than HV	rating? (Separate HV PSU and RF decks)
> 
> 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 at contesting.com 
> > [mailto:amps-bounces at contesting.com] On Behalf Of Marius Hauki
> > Sent: Tuesday, September 14, 2010 4:46 AM
> > To: amps at 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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