The electrician has returned to his point that the fuse won't protect against long-term overload.
As he said before: "that cable will be unprotected to sustained overload & long term damage despite being protected by a limiting fuse. Fuses work fine in a short circuit but in an over load can take as much as 1.8 to 2 times there rated current & there have been cases where this has been even higher."
He now continues:
"The fuse protecting the cable between the socket & FSU only has potential to blow quickly under short circuit conditions, under over load it will carry twice its value 26a+. The way you have it wired the FSU is the one & only accessory allowed from the ring radially regardless of what you wire downstream of the load side of the FSU. The potential for a dangerous situation to occur is evident to see, the regs are there to follow & prevent this."
Leaving aside the question of why he thinks it makes a difference whether the cable is before or after the FCU, we've dealt with that.
The cable is 2.5mm T&E clipped direct, so I believe that is rated to 27A under BS7671, so actually covers the double-rating of 26A that he is talking about. But that makes me wonder about the double socket that was there before, installed by the housebuilder (which I have replaced with an FCU).
If a 13A fuse can allow greater that 13A under a slow-overload condition (which I think is right) then the double socket could theoretically have been drawing more than 26A (via two plugs each with 13A fuses and each running overloaded above 13A, but not enough to blow the fuse).
Or to put it another way, if I now wanted (which I don't!) to install the same T&E with installation method 103 "In a stud wall with thermal insulation with the cable NOT touching the inner wall surface" which is then downrated to 13.5A, would the 13A fuse be too high to protect that cable, given a slow overload could occur?
As he said before: "that cable will be unprotected to sustained overload & long term damage despite being protected by a limiting fuse. Fuses work fine in a short circuit but in an over load can take as much as 1.8 to 2 times there rated current & there have been cases where this has been even higher."
He now continues:
"The fuse protecting the cable between the socket & FSU only has potential to blow quickly under short circuit conditions, under over load it will carry twice its value 26a+. The way you have it wired the FSU is the one & only accessory allowed from the ring radially regardless of what you wire downstream of the load side of the FSU. The potential for a dangerous situation to occur is evident to see, the regs are there to follow & prevent this."
Leaving aside the question of why he thinks it makes a difference whether the cable is before or after the FCU, we've dealt with that.
The cable is 2.5mm T&E clipped direct, so I believe that is rated to 27A under BS7671, so actually covers the double-rating of 26A that he is talking about. But that makes me wonder about the double socket that was there before, installed by the housebuilder (which I have replaced with an FCU).
If a 13A fuse can allow greater that 13A under a slow-overload condition (which I think is right) then the double socket could theoretically have been drawing more than 26A (via two plugs each with 13A fuses and each running overloaded above 13A, but not enough to blow the fuse).
Or to put it another way, if I now wanted (which I don't!) to install the same T&E with installation method 103 "In a stud wall with thermal insulation with the cable NOT touching the inner wall surface" which is then downrated to 13.5A, would the 13A fuse be too high to protect that cable, given a slow overload could occur?