The thermal cut out offers a very different protection to  an RCD, and that RCD may be either as part of the extension lead, or as a separate plug in unit, or not needed if it is  provided by the fixed wiring of the building if rewired in the last decade or so.

The thermal cutout protects against overheating - folk plugging things in without uncoiling the lead, and not realizing that this reduces the ability of the cable to 'sweat off' the heat, and lowers the maximum safe current by a large factor (a reduction in maximum safe load by as much as 3:1 in the centre of a big cable drum ).

The RCD provides automatic disconnection in the event of a different type of fault situation, where otherwise someone may get a shock from damage to the lead or equipment.

Long leads are more problematic, for a number of reasons - there is more length of cable to get damaged of course, they are more likely to have one end or the other taken outdoors , perhaps into a wet environment, and then the resistance of the cable itself means that fuses will not blow so fast, and that layer of protection is reduced.

For all these reasons an RCD is a very good idea with a long extension lead, and if there is any doubt at all about there being one as part of the building wiring it is plugged into or not, then the safe thing is to have another, either attached to the lead or as a plug in unit.

In terms of your PAT, by far the most useful test is the one where the cable is unwound and inspected for damage to the outer jacket. Anything that exposes the inner coloured cores should be considered a fail.

If you wish to fool your tester into doing the IR test, then you could 'pass' the resistance on another lead, and then swap the to the one you need to IR,  I suppose, but it may be worth asking Seaward if there is a  more elegant solution.

Mike

Many thanks Mike, that's very useful advice. I always carry out a full visual inspection before running the electrical tests, but I may try the IR trick using another lead, as you suggest. I'll definitely be checking whether my colleagues are using a plug-in RCD, but will offer the option of attaching an RCD, in case folks forget to use one! 

Best wishes, Becky 

An extension lead of that length should in my view be fitted with a permanently wired RCD plug, in place of the existing plug. I have little faith in plug in RCDs as they are vulnerable to loss, theft, or not being used.

I am not familiar with the instrument you are using but I suspect that you have selected an automatic class 1 test which will be impeded if the earth test fails. I would be surprised if the instrument did not allow for separate individual tests to be conducted.