Isolation transformer

[Bushbaby]

This is the 2nd one of these 550w RS isolating transformers that I have (this is one I am using in the garden workshop). I've always had these wired with a 3amp fuse and its is connected to an a mk rcd twin 30mA socket in use. Since I have been using this recently for some refurb work (my WEM Custom 15) I have noticed that it will occasionally blow the fuse in plug without tripping the RCD. Does this suggest that I have a problem with the transformer OR just that the initial power input is just too much (connected to a valve amp) and causing the 3 amp fuse to blow? I've attached a pic of my set up.

Many thanks

Rob

   

[ppppenguin]

The starting surge from a 500VA transformer can easily blow a 3A fuse. Since there isn't any inbalance between live and neutral currents, casued by leakage to earth, the RCD will not trip. Remember that using an isolating transformer removes any protection given by the RCD.

[Bushbaby]

Many thanks
As a safety professional I should have realised that the isolation would not have tripped the rcd (doh!) Am I better off doing all of my work through the rcd's than the isolation transformer?
I'm also using a 10mA single MK socket that was given to me when I used to work as a research chemist many, many years ago (it was brand new and left in its box for over 30 years before I started using it).

[ppppenguin]

There have been several discussions about RCD vs isolating TX here and elsewhere. There is just one main reason for using an isolating TX: To connect earthed testgear to live chassis radios etc. This is also relevant when working on the mains side of switchmode PSUs and for the same reason.

Otherwise an RCD gives you a lot of protection if you touch a live conductor. A 10mA RCD will give you greater protection but at the expense of more nuisance trips.

Neither TX nor RCD is a panacea, nor a subsitute for working safely. For example in a radio with double wound mains TX and earthed chassis you can still get a nasty belt from the HT. Neither isolating TX nor RCD will protect you from that.

[Murphyv310]

In fact a DC belt from HT is thought to be more dangerous than AC mains.
You are more likely to have reflex action on AC to react and come off than DC. My 1.5kv isolating transformer would occasionally blow a 10 Amp plug fuse but is OK with a 13 Amp fuse. There is no RCD on prior or after the transformer as it's superfluous in that situation.
On AC/DC sets or SMPS the use of an isolating transformer is mandatory even in the HSE documentation for the Radio & TV trade. One place I worked for (Brands that went bust in 2002) only had RCD protection for the benches (metal based and earthed) which resulted in numerous belts even with the RCD. Their H&S officer was allegedly trained with a certificate, I told her one did she get the certificate out of a "lucky bag"!! Two of us called HSE after one guy got a belt and split his head after falling. They came in and immediately closed the place until isolating transformers were installed.

[Bushbaby]

Thanks for your replies.
I shall continue using the transformer for most work and I think its worth upping the fuse to 5A and see what happens.

Best Regards

[ppppenguin]

I usually use an isolating TX with live chassis sets, simply because I routinely want to use my oscilloscope with problems. I'm well aware that it negates RCD protection. If I touched the HT on such a set without the TX, I'd get a belt but the RCD would trip to ensure it was a very brief one. With the TX it would be like touching the HT on a set with a double wound mains transformer. I'd get the belt and nothing would trip.

Actually it's more nuanced than that. If I'm using the isolating TX and no earthed kit is connected I can touch any part of the set, including HT and get only a minor belt due to my own capacitance to earth.