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Hope we get there soon John 'cos the forum is running out of storage space! Have a look at OMS's answer to my query on the IET forum - it helps a little in a parallel way. Regards

John, The discussion has always been on track and, due to the discourse that has taken place and feedback you have received, you have revised the figures you believe acceptable for minimum Zs downwards from 0.63 to 0.28 ohms in your latest table (in the case of 1+1 T&E). Therefore, you must...

Of course the suggested plasma forms within the MCB! I am getting the distinct impression that I should not continue here. Goodbye

John, You'll have go ask much higher authorities than me. I have read that, below 0.01 seconds, the energy transfer is considered to be non-adiabatic. That is, heat is involved due to the fact that a plasma field develops. Cannot remember where I saw it and do not know whether itsis correct...

John, Although it can be inferred from the way I have written my last reply that I believe that the adiabatic only applies between 0.1s & 5s you will know from all my previous arguments that this cannot be what i mean. I have used the adiabatic and confirmed the derivation of Coates and...

The way I see it is this:- For MCB providing fault protection only, extra calculations are necessary. I take it it that (please correct me if wrong) in the disconnection interval where the time is stated to be 'instantaneous' for an MCB (I take this (or had) as between 0.1 second and 5...

Not ignoring you EFLI - all points noted. John, do not quite understand - you have put forward a table which states that the minimum Zs for a 2.5mm/1.5mm cable is 0.46 ohms at 230V. I have put forward an example where Zs = 0.26 ohms and asked whether this is safe. You then argue that you...

Have done some googling - the nearest I can find is that 0.01 seconds represents one half cycle of sine wave mains - below that (and above 5 seconds) the equation becomes non-adiabatic and energy let through calculations are considered more appropriate. Understanding that using the 0.1 sec...

Sorry - repeat post.

Much more experienced people than me have stated that the design step uses the formula 0.1 Uo/kS - to determine minimum Ze at the MCB - that is Coates and Jenkins. Surely we should try to understand why they say that? Unless I have misinterpreted/misapplied? Regards

That's it John. My reasoning is that, if Coates and Jenkins have chosen 0.01 as the definitive minimum disconnection time, then why don't we? As you have implied, the choice of which disconnection time is used is seemingly arbitrary - from Appendix 3, why not 1 second or 10 seconds? So, I...

John, It may be, but I am basing my calculation on a formula and discussion in Electrical Installation Calculations (Coates and Jenkins) Fourth Edition - page 114 and 115. Assuming that the definite minimum time is 0.01s , the minimum value of Zs at the MCB is given by: 0.1Uo/kS and...

Hello John, Spent some time looking at this (please check all my calculations) and using books available to me:- At 253V, for 70ºC sheathed cable the minimum Zs that can be tolerated at the MCB to provide protection to the CPC is 0.1 (V)/kS = 0.1(253)/115(1) = 0.22 ohms for 1mm² T/E...

Though at first glance, do we need to use 230V as the nominal voltage? Regards

Hi John, This looks very interesting. Will have a look in a week or so, as am away with the boys. Regards

I would recommend Electrical Installation Calculations by Coates and Jenkins and Commentary on IEE Wiring Regulations by Paul Cook as well as the title you endorse. To obviate the problems associated with unskilled design, I think that the powers that be designed the OSG as a 'coverall'...

I try to avoid the OSG as it makes generalisations. I think it bases its assertions on standard circuits and that OPD are providing overload and fault current protection. As such, the circuits in the OSG should be inherently safe. With regards your postulate as to whether the subject is known...

Seems like you have it all covered John. Regards

RCD's would normally be expected to protect cable in respect of fault to earth but you would still need to protect cable for the time it takes the OPD to clear short cct's between line and neutral, line and earth - particularly in areas where there is high PSCC or high PEFC. Have a look at...

434.5.2 ? Regards