Discuss levels of rcd protection on TT system - long post - prepare cuppa first! in the Electrical Forum area at ElectriciansForums.net
Yes you are quite correct but in agricultural it is the difficulties of bonding extraneous-conductive-parts.
i find the guidance notes are the most helpful when trying to decipher bs 7671.There is exposed structural steel in the packing shed, doors are metal too, can easily get a 10mm earth to it if its acceptable to be on pme this is all that is required accross the site, or do i TT the lot. If the 100ms upfront does satisfy that reg 705.411.1 (iii) and discon times arent relevant then thats doable too. Why is this bloody regs book so open to interpretation!!!
If you can get a 10.0 cpc across that would be my choice.There is exposed structural steel in the packing shed, doors are metal too, can easily get a 10mm earth to it if its acceptable to be on pme this is all that is required accross the site, or do i TT the lot. If the 100ms upfront does satisfy that reg 705.411.1 (iii) and discon times arent relevant then thats doable too. Why is this bloody regs book so open to interpretation!!!
It may not be in section 705 but disconnection times are disconnection times.bottom of page 24........ pme not recommended unless a metal grid is in the floor! ffs I think I'm going to price to TT the bitch and be done with it.
Can you chaps just answer me the question of whether I still need to meet my 0.2 sec discon times for the 32A tp panels? as its not in section 705 - just says rcd ratings.
installing 30mA protection on each one would at least reduce the likelihood of the incoming type S tripping - if ever, as it stands now a decent fault on any of the pumps has the ability to trip the entire site.
ps thanks everybody for their input, including the hijacker! all aids the cause
The RCBO's will not give discrimination over the Selective RCD as they are only single pole therefore a neutral to earth fault on a tripped RCBO will still trip the main RCD if sufficient current is flowing in the system to divert down the neutral earth fault !
Because the Neutral is common and not isolated in the event of a fault it could still could cause s type to trip yes but I'd like to think the 30mA will disconnect any current in the neutral first in the faulty circuit before the time delay trips.The RCBO's will not give discrimination over the Selective RCD as they are only single pole therefore a neutral to earth fault on a tripped RCBO will still trip the main RCD if sufficient current is flowing in the system to divert down the neutral earth fault !
Requirements are that the line and neutral both need isolating. A 2 pole rcd or 2 pole mains switch on single phase systems will account for the double pole isolation on a TT as it's an acceptable method and will satisfy the requirements.By the way my personal opinion is that single pole RCBO's should not be allowed on TT systems, after all the regs says that both live and neutral should be treated as live on a TT system !
Reply to levels of rcd protection on TT system - long post - prepare cuppa first! in the Electrical Forum area at ElectriciansForums.net
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