I'm not sure if those old Heinemann RCD's had overload protection, I suspect not because there's an overload main breaker (MCB) to the left of it so that would be duplicating overload protection if it does. The 30A it states on the front might be an indication of the maximum load it's rated to be able to disconnect at if there's an earth leakage fault. Maybe another member here has a better memory than me and can confirm if it has overload protection.
Earth leakage is a cumulative problem and even more so with a 3-phase RCD. It sees the total leakage on all 3 phases and if that total leakage exceeds 30mA it trips. Generally speaking it's better to have 3 separate single phase RCD's, it's causes a lot less inconvenience if there's a tripping fault and that alone may be good enough justification to replace it assuming there's space in the board for 3 single phase units and 3 associated neutral bars. As you say earth leakage tripping faults can be confusing if you're trying to solve them by deduction, I made a thread somewhere with advice for homeowners with tripping problems, I'll see if I can find it later and post a link here when I've got more time.
I feel for you and you're in the same boat as many home owners, sometimes they've unknowingly been let down by unscrupulous contractors and sometimes they're to blame for knowingly cutting corners to save money. I'm afraid the electrical legislation holds the home owner as being the person responsible for the electrical installation and its compliance.
3kA would usually suffice, especially on a 40A main supply but it could be higher if for example you happen to be close to the Eskom transformer or if for some reason you have an oversized supply cable. You could enquire with the municipality but it might be a tedious process, the best way to be 100% sure would be to test the PFC which would take a sparky with the right test equipment literally 2 or 3 minutes.
Thanks for the offer to engage my services but I'm afraid I'm not set up to do domestic electrical, primarily I design and build control panels for the commercial market. I'm also located near Noordhoek which is nearly an hour from Tableview. Maybe take up SeanM on his kind offer of a free test, I'm sure he'd also give you some good advice on your best way forward as well if he sees the DB first hand.
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