Definition of high protective conductor currents

There is another approach which completely removes the issue, localising earth leakage to the heat pump itself. That is to use a transformer with simple separation.

Example for single-phase in Figure 12.4 (p104) of GN5.

If manufacturers built that into the heat pump, or used an inverter with separation (possibly cheaper transformer like in a DC SMPSU), no special precautions are necessary in the installation.

That is to use a transformer with simple separation.

Very true, but to a manufacturer, quite undesirable for a number of reasons. The obvious penalties are size and weight, although that only increases manufacturing, materials, storage, and shipment costs. Then there is also a problem in terms of selling the efficiency ratings to the user (not just explaining an increased floor or height claim) for reasons of transformer losses once in use - its an extra few % of the kVA going as heat that has to go somewhere - trivial if its a few % of less than a kilowatt, but actually really awkward once you get to  using VSDs that are ten times that or higher. 

It is for the same reasons that it is now almost impossible to buy any domestic appliance that does not contain a switch mode supply of some kind - and the UKCA/ CE efficiency requirements have all but removed traditional linear power supplies even from the small 'wall-wart' applications, so the modern house is buzzing with RFI generating switchers.


And of course its not the maker's problem to say 'use an RCD as required by local regulations', however unhelpful it may be to us, it's very easy for them, the extra ink in the handbook is almost free.

So while the transformer idea is technically appealing, and personally I'd love it , I'm not holding my breath for many makers to adopt it.
Mike.

mapj1 said:

And of course its not the maker's problem to say 'use an RCD as required by local regulations',

I'm led to understand some inverters (e.g. heat pumps) need an RCD with a performance that doesn't yet have a classification in standards (but does exist in the market place).

On top of that, the cost of even a standard Type B is an issue of cost to someone, as well as the fact that it may also use power (but that may well be less than the losses of a transformer).

mapj1 said:

Very true, but to a manufacturer, quite undesirable for a number of reasons. The obvious penalties are size and weight, although that only increases manufacturing, materials, storage, and shipment costs. Then there is also a problem in terms of selling the efficiency ratings to the user (not just explaining an increased floor or height claim) for reasons of transformer losses once in use - its an extra few % of the kVA going as heat that has to go somewhere - trivial if its a few % of less than a kilowatt, but actually really awkward once you get to  using VSDs that are ten times that or higher. 

Perhaps another approach could be to use a DC-DC converter stage with HF separating transformer, between rectified AC input, and inverter output, to provide the separation, and drastically reducing the impact of providing separation on losses, cost and weight!

gkenyon said:

Perhaps another approach could be to use a DC-DC converter stage with HF separating transformer, between rectified AC input, and inverter output, to provide the separation, and drastically reducing the impact of providing separation on losses, cost and weight!

Or just design out the need for a RCD of any kind. Appropriate design would negate the need for RCD protection on TN systems or additional protection on TT systems. 

gkenyon said:

Perhaps another approach could be to use a DC-DC converter stage with HF separating transformer, between rectified AC input, and inverter output, to provide the separation, and drastically reducing the impact of providing separation on losses, cost and weight!

Or just design out the need for a RCD of any kind. Appropriate design would negate the need for RCD protection on TN systems or additional protection on TT systems.