Construction sites and the use of RLV, guidance on the use of residual protection?

 RCDs on 110v sockets

from the discussion above, which is a few years ago, a conclusion was made that RCD’s are not required for RLV/ CTE transformer used in construction sites from Sec 4. 
It is clear from section 4 that additional protection for RLV sockets is not a requirement, for obvious reasons CTE 55v etc. 

however I have noticed a more recent trend of RCCB’s integral to the secondary side of the windings being implemented to ensure fault protection that can clear within the 5 second disconnection time under 411.8.3. It would seem primarily this is due to chaining multiple extension leads together or excessive lighting circuit lengths, which are difficult to account for except by direct supervision on site. 
Blakley, being one of the leading manufacturers offer a great deal of good technical guidance on the subject - but largely it would appear the practice of having both RLV + residual protection combined into one transformer is down to preference rather than a requirement of BS7671 or BS7375 for that matter, meaning this would be ‘best practice’ as apposed to mandatory unless a contractor were to make part of their policy. 
I would be interested in the forums opinions on this subject, and your own experiences, I think there are changing trends in the industry as to what was historically an acceptable risk may no longer be the case, particularly I’ve noticed 110v Tx plugged into blue commando sockets with a 30mA RCD protecting the primary side, I’m unsure what degree of protection this will offer to outgoing artic flex on the secondary side and it doesn’t seem it would be the same as a multiple pole device that’s installed on the secondary side?

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  • As you say it is TNS, referenced to earth through installation earthing arrangement and would remain in place even if the earthing system is not effective in the main installation.

    That is if the earth is actually used. Yesterday afternoon I was using my 9" 110 volt angle grinder plugged directly into the 32 amp socket on one of my tool transformers to cut through a concrete path.

    The Bosch angle grinder has a two core 2.5 mm flex, if I had knicked the cable and exposed a live conductor the chances of establishing an earth path that would ensure the correct operation of the circuit protection inside the transformer a fuse is nil, so is the likelihood of tripping an RCD, but the voltage would be less than 55 volts.

    Had I severed the cable with the steel cutting blade and shorted the two conductors, then the fuses might blow, but only one of them might rupture leaving a live connected. 

    However, I did carry out user checks on the equipment and I ran my fingers along the flex checking for damage, as well as looking at it and the power tool.

  • Not that this is especially unusual or bad - a 240V angle grinder may well also have a 2 core lead, and the cable damage scenario is the same, except the maximum exposed voltage may be nearly four times higher,  and fuses don't always 'let go' when a mains cable is cut anyway - I have seen both lawnmower and hedge clipper leads cut with the fuse intact and the power on, and there are plenty of situations dry enough that the current that does flow is so limited an RCD will not operate - but then that limited current is not dangerous in that situation,

    As you say, the important thing is an alert user.

    Mike

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  • Not that this is especially unusual or bad - a 240V angle grinder may well also have a 2 core lead, and the cable damage scenario is the same, except the maximum exposed voltage may be nearly four times higher,  and fuses don't always 'let go' when a mains cable is cut anyway - I have seen both lawnmower and hedge clipper leads cut with the fuse intact and the power on, and there are plenty of situations dry enough that the current that does flow is so limited an RCD will not operate - but then that limited current is not dangerous in that situation,

    As you say, the important thing is an alert user.

    Mike

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