On Thu, 24 Dec 2009 14:43:30 +0000, Ian Bell wrote:

I am trying to select a relay for a delayed HT switch (which will also
discharge the HT when off). Most relays I can find have contacts rated
at 250VAC which translates into a peak of about 350V. However, data is
scarce on what dc voltage these relays can switch. So far I have found
only one that gives a dc current versus voltage curve and that stops at
210V dc (and 200mA) and I really want to be able to switch up to 350V at
up to 200mA. The rest just give a dc voltage at max current value.

The reason DC switching ratings are lower than AC ratings is that, when
breaking a circuit, AC limits the duration of arcing as the current
through the contacts drops to zero twice a cycle thus helping to
extinguish the arc. With DC the contacts have to open wide enough to
extinguish the arc on their own.

You say:

The PSU is remote and the relay has an interlock to turn off the HT if
the PSU HT output lead is disconnected. I need to either disconnect the
HT or bleed it very quickly to avoid a possible shock hazard.

I assume we have the following setup in the PSU:

relay
HT+
to -------------o COM
LOAD /
o o----------- HT+ SUPPLY
NC | NO
|
-----
| R | - discharge resistor
-----
|
GROUND -----------+---------------- GROUND

The relay contacts change from NC to NO a little while after mains has
been applied to the PSU (allowing valves time to warm up) and from NO
back to NC when either the system is switched off or the HT output lead
is disconnected.

In the first instance there is no current flowing until the contacts
close, so no arcing to bother with and I wouldn't worry about the relay's
DC switching rating being lower than the HT voltage: as long as the relay
is capable of holding off the voltage when the contacts are open and no
current is flowing (and of handling the expected supply current) it
should be OK.

In the second and third cases the HT current will try to arc across from
the NO to the COM contact until COM closes on NC at which point the
discharge resistor should attenuate the voltage from the supply and
extinguish the arc. In the second case - when the power to the PSU is
switched off normally - if you can allow a certain delay before operating
the relay then most the the HT energy should have been dissipated in the
load electronics, so the relay should have little current-breaking work
to do and the contacts would not be strained.

The third case - when the HT lead is disconnected - seems to pose the
most stressful conditions. The interlock that operates the relay must
also cut mains power into the PSU, and it is then a case of whether the
relay can handle the energy stored in the transformer and smoothing
capacitor(s) at the moment of switching. However this situation should be
relatively infrequent so as long as the relay contacts don't melt down
instantaneously it shouldn't be a problem.

I think your plan to choose a relay with contacts rated at higher current
than that you expect to switch is good: the relay will in any case need
to be rated to supply the current from the supply (with fully charged HT
smoothing capacitor(s)) into the discharge resistor. A bit of suck it and
see with deliberate disconnection of the PSU from the load and
observation of the relay as it operates (assuming it has a transparent
cover) would be the order of the day.

--
John Stumbles

When I die, I want to go peacefully in my sleep like my father did,
not screaming in terror like his passengers.