Earth loop Impedence

It would help to know what circuit breaker protects the circuit - see

I take it this is just a basic go / no go style tester?

In which case telling you that the ELI is under 1.7 ohms will give you confidence that the earth is connected to the socket, and is in all likelihood "ok".

It can't really go much further than that without giving you an exact reading. Given an exact reading, then you could interpret it further with knowledge of the circuit and its protection, to work out if the result is good enough.

To interpret it further you would need to know the protective device on the circuit, and also how far you are from the furthest point on the circuit away from the supply. So for a ring circuit with a B32 MCB on the head end, you would need an ELI of no more than 1.37 ohms at the furthest socket from the CU (as the cable runs - rather than actual linear distance)

Yep:

Ah, OK, thanks. Of pretty limited use, then!

It gives the same 0-1.7 range reading at each socket - maybe 25m cable run to the most remote. Sockets are protected by 32A MCBs and an

Its worth having IMHO... basic testers will tell you a socket is earthed, but will not spot situations where that earth is actually very poor and functionally useless. Seeing something that also give a feel for what the value is (even if in broad categories) give better confidence.

Yup, you would expect less than 1.7 ohms on any socket on that circuit (assuming its not a TT earthed installation). In fact any indication other than the lowest would be cause enough to investigate further.