Over twenty years ago I did a job that had a similar device that tested fine, but it was rated at 30 mA rather than 500 mA like this device and I would have been using a Robin KTS1620 tester which is not as subtle as the new testers, it’s a bit more akin to a sledgehammer.
Missing from the simplified diagram for the tester is the fuse or breaker in series with the 50v winding, so that if you do have a TN system by dint of bonded plumbing or similar, there is something to protect the transformer from total destruction (or perhaps a high wattage resistor and a warning lamp).
But that is not a VO ELCB - that really is an early RCD, or CO ELCB as first known - the give-away is the absence of the pair of earth connections, one for the installation CPC and one for the terra-firma electrodes - shown as F and E in the lower diagram.
Still it is indeed time to change it, perhaps for a 100mA time delay or a 30mA one, depending on the RCD arrangements for the rest of the set-up.
