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AFDDs & EVCs

Eddie Currents
Question 1 - AFDDs 

We are looking at installing combined AFDDs/RCBOs to existing switchrooms located in some High rise flats starting with the landlords supplies then moving on to the consumer units located within the flats themselves. The issue with the landlords supplies is that there is no manufacturer that appears to make the AFDD bigger than a 40Amp range and also they are unavailable in 3 phase. Therefore if we carry this work out then a number of circuits eg where you have a 63Amp 3 phase lift supply then this will need to be mounted in another seperate enclosure with cct fed via an MCB thus we are only doing half a job - any ideas ? EATON suggest using 3 seperate single AFDDs to protect a 3 Phase circuit but for me this is bad practice as it means 3 points of isolation in the one Db just to isolate the 3 phases serving the one T/Pole Lift cct !!!


Question 2 - EVC's

We are looking at wiring 6 EVC points rated at either 11kw -22kw. They shall be fed from a public building in new underground ductworks. The building has it's own dedicated sub station believed to be a TNS. Because the Regs and COP make no ref to TNS I assume the EVC Charging points will not need any independant earth spikes ? Just to clarify that each EVC point cct will be protected at a dedicated Db located inside the building each fed by independant Type A rcds. The EVC Parking spaces will be located in the buildings shared car park adjacent the building 40m away. There is some existing column lighting in the car park  which is fed from the opposite building which has it's own transformer. If we locate the EVCs more than 2.5m from any existing column lighting and street furniture fed off the opposite buildings supply then surely this will suffice ? Hoping its not a showstopper ? 

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    Personally I'd dump the idea of fitting AFDDs - they are unproven technology and despite no being required under the wiring regulations, are presently being much hyped by the manufacturers.

    Eaton would most certainly be delighted to recommend that you fit 3 of them - they are only designed to protect one circuit at a time and are hideously expensive, and little empirical evidence has been demonstrated that they actually work with all arc signatures..

    The technology is still in it's infancy and early adopters will pay the premium for being the guinea pigs for actual site testing.
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    Fitting AFDD to the power supplies of lifts in high rise flats that may be used in an emergency by the fire brigade to access the building and residents to evacuate from it sounds like something that should definitely be reconsidered to me.


    Andy Betteridge
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    I think both are good points. I think the client could be panicking a little to get them in quickly. However the buildings have sprinklers and L1 fire alarm systems in place so may be overkill protecting all circuits with AFDDs. Maybe just the flats themselves and all Landlords power and Lighting circuits shall suffice (not the safety circuits)  ?
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    Do the safety circuits including the lifts have RCD protection rated at 100 mA or more to reduce the risk of fire due to a fault on that electrical circuit?


     Andy Betteridge
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    No they are just protected by MCBs
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    Installing RCDs rated at 100 mA or more is done to provide fire protection.


    However installing a 300 mA or 500 mA RCD to the lift supplies would send most people into melt down worrying about what would happen if there was a nuisance trip, particularly during an emergency.


    I think you need to apply the same train of thought to AFDD on safety circuits including lifts, what if there is nuisance tripping during an emergency?


    Increased periodic inspection and testing to detect continuity or insulation faults seems preferable.


    Andy Betteridge
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    Fandango:

    Question 2 - EVC's

    We are looking at wiring 6 EVC points rated at either 11kw -22kw. They shall be fed from a public building in new underground ductworks. The building has it's own dedicated sub station believed to be a TNS. Because the Regs and COP make no ref to TNS I assume the EVC Charging points will not need any independant earth spikes ? Just to clarify that each EVC point cct will be protected at a dedicated Db located inside the building each fed by independant Type A rcds. The EVC Parking spaces will be located in the buildings shared car park adjacent the building 40m away. There is some existing column lighting in the car park  which is fed from the opposite building which has it's own transformer. If we locate the EVCs more than 2.5m from any existing column lighting and street furniture fed off the opposite buildings supply then surely this will suffice ? Hoping its not a showstopper ? 

     




    If it's guaranteed to be a TN-S supply, this is OK for the EV charging equipment


    BUT


    It's not just the EVSE (charging equipment) you need to keep out of arm's reach (2.5 m) of the lamp posts ... perhaps more important the vehicles on charge must also be at least 2.5 m away !!!


    You can't I'm afraid simply bond the two systems together without knowing the implication on the earthing systems. Incidentally, these are "general rules" of BS 7671, and NOT Section 722 specific (in particular the requirement about simultaneously accessible exposed-conductive-parts is Regulation 411.3.1.1 (my highlight):



    411.3.1.1 Protective earthing

    Exposed-conductive-parts shall be connected to a protective conductor under the specific conditions for each type of system earthing as specified in Regulations 411.4 to 411.6.
    Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.

    Conductors for protective earthing shall comply with Chapter 54.

    A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point.



    And you can't simply bond the two installations together because of regulation 542.1.3.3 (what value of fault current do you consider ... and would it also be HV fault current).



    In terms of RCD protection, is Type A sufficient? OK if there's an RDC-DD device either in the charging equipment or downstream of the Type A, otherwise Type B is required.


    This is all covered in the IET Code of Practice for EV Charging Equipment Installation which I would strongly recommend you have a good gander at before assessing this particular job.

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    Thanks Graham - much valued information as always, will take a good look at the IET COP before carrying out the appropriate designs.
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    I was pondering the installation of AFDD to safety circuits.


    Several years ago I install a circuit in FP200 fire rated screened cable to supply a domestic fire circuit pump in the cupboard under the sink with sprinkler heads mounted into the kitchen tap and on the wall of the adjoining dining room.


    It was a very simple circuit with a dedicated CU in the cellar by the intake, but it had to have a 100-mA RCD upfront, because it is a TT installation along with a C type MCB.


    The only time the circuit will ever be fully loaded is if the pump kicks in, because the house is on fire.


    It seems rather daft to consider installing an AFDD to a circuit like that as the only time it could identify a series arc that could start a fire and shut the circuit and pump down is when the house is already on fire. Given the cable construction a parallel arc should trip the MCB and probably the RCD as well.


    It does seem to me that fitting an AFDD to such a circuit would be completely counter productive. 


    The AFDD needs to protect the rooms people are sleeping in, not the safety circuits.


    Andy Betteridge.


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    Totally agree Andy - the regs state AFDDs in sleeping accomodation, places of high value or buildings made of combustible material. How we interpret the Landlords circuits in high rise flats is open to a verdict. If only the regs were much clearer !!!

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