[Amps] Switching off Drake L-4B occasionally trips mains RCD

[Tony Brock-Fisher]

Using the US terms, a GFI or GFCI senses the current in the hot and neutral 
leads to a device, and trips when they are not equal. The idea is any 
current that traveled to a device on the hot lead and did not return on the 
neutral lead might be finding a path to ground through a human.

First, look for a path or circuit which might be sending a current spike in 
the neutral lead when the switch is shut off. Perhaps the collapsing 
magentic field in the transformer, or from the fan, is enough to cause a 
momentary current imbalance of just the right phase when the switch is 
opened.

Another simple experiment would be to make up an extension cord (male/to 
female plugs) with a double pole switch in it that breaks both sides of the 
line. When powering down, open this switch first and see if it avoids the 
problem.

-Tony, K1KP

----- Original Message ----- 
From: "Alan Ibbetson" <alan at g3xaq.demon.co.uk>
To: <amps at contesting.com>
Sent: Wednesday, October 03, 2007 5:25 PM
Subject: [Amps] Switching off Drake L-4B occasionally trips mains RCD


> As is common in the UK, the mains supply for my whole house is protected
> by an RCD (see http://en.wikipedia.org/wiki/Residual-current_device for
> a description). I think they are called GFCIs in the US.
>
> Every once in a while, say 3 times this year, this RCD trips when I
> switch off my Drake L-4B amplifier. Murphy's Law dictates that this will
> always be at night, so the house is plunged into darkness, and in the
> middle of the XYL's favourite soap, so I am plunged dans la merde.
>
> Tonight was one of those nights and I'm seeking help on finding the
> cause and curing it. Reproducing a fault that only occurs a few times a
> year is going to be problematic, so I wonder if anyone has any
> hypotheses as to what is happening?
>
> For those unfamiliar with the L-4B, it only uses a single pole switch
> for the mains power, can be wired for 115v or 230v AC with jumpers on
> the transformers, and uses the tapped primary of the filament
> transformer as an auto transformer to supply 115v to the blower when
> operating from 230v mains. I had problems with heat with the stock
> blower so I fitted a Dayton 4C761 as advised on this list. I always have
> the mode switch on the lower HT CW/Tune position when I switch off. I
> don't know whether any of this is significant, I'm just trying to
> provide you all with data.
>
> Ideas anyone?
> -- 
>
> Thanks,
>
> Alan G3XAQ
> alan at g3xaq.demon.co.uk
>
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