I am visiting a customer today to quote on installing an outside socket for the supply of a split air conditioning unit.
Installation manual just says, recommend the use of a 30mA RCD, no other details.Phoned tech support, apparently never been asked what type of RCD should be used and didn't know there were different types. ,Just know that it should be a high immunity RCD. Need to find out if the high immunity RCD requirement is just general guidance or for specific models. The unit creates a variable load between 0.5 and 7.0A, with max current of 10A, presumably on start up. The variable load implies to me a variable speed drive.
There are a few companies that provide high immunity RCD's, Hager, Schneider etc. Assuming that the convenient way to supply the outside socket will be from an existing ring final it's going to be inconvenient and expensive if I can't use the existing RCD or RCBO and a suitable replacement isn't available, clearly if there's a type AC in place it will need changing.
Reading around I can see that high immunity RCD's tend to be type A's that have filters to stop tripping short spikes in leakage (up to 10mS) from tripping the rcd. It also sounds like a type F RCD has or may have similar features, couldn't find anything specific enough to trust. There are a lot more companies providing type F and type B RCD's now, hence the interest in suitability.
I could be very safe and completely over engineer the job, but then a handyman will just quote for a external socket on an existing type AC RCD. I loose the revenue and customer has an unsafe installation. Trying to find the right compromise.
As a side question I am trying to get a better understanding if RCD types and in practical terms what improves when going through type A, F and B. Clearly understand the DC blinding issue, but as we move beyond type A, as an example are we gaining immunity to high frequency leakage or are we gaining the ability for high frequency leakage to cause a trip. If I spend long enough reading the specs I will work it out, just wondering if there is a good explanation anywhere.
Looking at BS7671 it has a table showing what leakage from various AC rectifier faults look like. But there doesn't seem to be anything showing what leakage from switched mode PSU's, variable speed drives etc looks like under normal and fault conditions. I can make some educated guesses, but overall feel there's a lack of information.
