Where do they pluck the max values from, I have just done a test and inspect on a property and it has a 3 phase board that has 2 x 63A TP MCBs feeding 2 buildings, one is wired in 25mm SWA and then other in 16mm SWA. The 25mm circuit has a type C in and the 16mm a type B Now the max Zs allowed for a C63 is 0.28 ohms, and that's going off a chart I have for 60898 MCBs, probably even lower for 60947-2s how can that be, the incoming Ze is more than that and at the other end I've got a Zs of 0.35, the figure given for a B63 is 0.58 This is using a chart with 80% values however, does anyone know the max Zs allowed for a hager C63A triple pole MCB I can write in the box. Hate form filling, joiners and brickies don't have any of this **** to put up with at weekend and get paid as much if not more than us.
Found a chart for schnider ones that lists theirs as 0.45 in worst case scenario, so thats whats going on the cert, **** it. http://pws.schneider-electric.co.uk...lByID/NKIY-5JUHSL/$file/MGD5032_section2.PDF/ Its amazing how I can turn a death trap into a safe install at the stroke of a keyboard, I feel like a part P council nazi now.
They pluck the maximum Zs values using ohms law and the instantaneous tripping current value of the protective device with Cmin taken into account for the fluctuation in voltage.
Hi Peter, If you check out Table 41.3 in the BYB page 58, you will find the Zs for a C curve 63A MCB is 0.35, with a Cmin of 95%. So theoretically, your circuit complies, given that you are Inspecting & Testing an existing installation. I assume you are talking about single phase circuits. Mathematically, a 63A C curve MCB has a time/current curve of 10x, which is 630A to achieve disconnection. 230V / 630A = 0.365 ohms,@ 95% Cmin = 0.3468 ohm, hence the 0.35 ohms in the table. You will also see that the B curve has a value of 0.69 ohms. DaveS
Cheers Dave, the slight problem I had though is the EYB on lists domestic MCBs (60898s) as far as I can tell. The MCBs in the board are 60947-2s, its a 3 phase board and the breaker is a 3 phase one. Also there is an 80% correction factor as well. However I got round that by just recording the 100% value, its only a figure on a bit of paper at the end of the day. My main issue was the Ze figure bandied about for a TNC-S install is 0.35 ohms, yes it may be lower than that on test but it could be slightly higher as well, and if its a TNS incommer then likely higher still. Yet they market an MCB that requires a Zs lower than what is probably coming into the building, how do they go on when the MCB is even bigger, using the biggest cable in the world along the shortest route wont achieve anything if its too high to start with.
You would select a different protective device in the design stage or if protected by an RCD then this could be used for fault protection and the mcb for over current protection.
It was just a bog standard 3 phase hager MCB, however I cant find the data for Hager MCBs so I used the Schneider one instead, how different can they be, its Sunday and I'm not devoting the whole day to pointless paperwork, although its currently 7PM and I'm still here.
So it was a C type Tripple pole 60898 mcb. Finding the manufacturers data might give you a little more leeway as could be anything between 5 and 10 x In for a type C. The 80% rule of thumb is to take in to account the difference between ambient temperature at 20 degrees C when testing and operating temperature at 70. The coefficient resistance of copper at 20 degrees C is 0.004, obviously the difference between 70 and 20 degrees C is 50 so 0.004 x 50=0.2 so this is where the 80% rule of thumb comes from.
Thought you couldn't use the 60898 standard in commercial situations and had to follow the 60947-2 type. Nobody ever reads them anyway and I'm not about to condemn something where there is a good chance I might be wrong, especially when they will believe the bloke in the pub before me.
It is an ET&I. It is not our job to question Ohms law. Record it as a non-compliance and move on. It is clear what you need to do if it is higher than the OCPD says it needs to be.
Not really a Zs thing, but a site I was on today had a eicr done a couple of months ago, hmo, no one resident since then. Found communal sockets in hallways with no real earth present, one of which had a neutral completely out of the terminal (rfc)-and that was just the start-beggers belief tbph, I reckon the guy, or gal-before we start to get pc-was just swinging it, not that that sort of thing actually happens, as we know...
But the standard MCBs have 60947-2 stamped down the side of them as well, why is it so complicated, all probabby dreamed up by the EU. I think to become an electrician nowadays you have to sacrifice your social life completely and become a geek with your head in a book, not so long ago a loop tester was a bulb and 2 crock clips and a continuity tester was a battery and a bell.
As per usual tbh, new owner of the property wanted revenue coming in as soon as!.....I installed new interlinked fire alarms, as per requirements for a hmo, a major water leak from a shower rendered an emergency light fitting inoperative, along with the rest of the lighting on that circuit. Two tails from a rfc breaker had been removed since my last visit, probably A.N.Other attempting to get the rcd for that side to reset. The new owner is doing the bear minimum to get tenants in, this also apparently manifests itself in weird and wonderful happenings, as previously mentioned, and yes it has been documented etc. But unfortunately I see this happening regularly, the electrics are only safe until a third party intervenes.
So assuming I put 80898 in the type box, what figure goes in the max Zs box, the 100% values as in the EYB or the 80% or cmin or whatever its called this week. And what would constitute a pass. This is all assuming someone elses loop tester doesn't give a lower reading by a couple of hundredths of an ohm.
It depends in what context you are using the 60898/60947-2 that will determine how you record it on the EICR. Is it a sub-main?
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