AM2 Max Zs
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Sharpend
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Iirc, They will ask you to measure Zs on sockets but calculate on other circuits. I think they ask for your figure to within the 80% range but can’t be a 100% certain as it’s a long while since I’ve been involved in the AM2.
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No idea about AM2...
But you must remember that on an electrical certificate it is the Max Permissible values from the big book BS7671, that you write in the boxes..
BUT.. unless you have taken account of the temperature whilst taking any cable test readings..
You want to be making sure your readings are no more than the 80% on-site-guide values..
In reality on a domestic installation it is unlikely to top the 80% values..
BUT.. if you do.. it is not the end of the world as long as you are NOT above the max chapter 4 tables values!
:coffee
But you must remember that on an electrical certificate it is the Max Permissible values from the big book BS7671, that you write in the boxes..
BUT.. unless you have taken account of the temperature whilst taking any cable test readings..
You want to be making sure your readings are no more than the 80% on-site-guide values..
In reality on a domestic installation it is unlikely to top the 80% values..
BUT.. if you do.. it is not the end of the world as long as you are NOT above the max chapter 4 tables values!
:coffee
also worth noting that you can exceed those in BS7671 if youre still within manufacturers permitted Zs. some are higher than 7671
True, although I've never seen any manufacturer state different values than BS7671 for a 60898 device? Old 3871 type 4 devcies are a different story, as the types were brought in later and the IM multiplyer had to be set really high at 50x to 'catch' all legacy devices, so you often find much lower manufactors figures, however you have to be careful, quite often they'll be relying on the thermal element to acheive disconnection times, which you can often see if you state the same (high) current value for some quick disconnection times, and then it starts dropping down with each step in time. The constant figure is the magnetic trip which has one value and then the values on the sliding scale are from the thermal element:
Take the 30A from above data, 700A at 0.02s and 0.1s and then goes to 360A at 0.4Sec, 120A at 5s. 700A is max zs of 0.31 ohms, really low for a 30A device, but still better than the 0.145 we would get from BS7671 figures (based on an IM of 30Ax50=1500A)
We only need to acheive 0.4s for our sockets ring though, so Zs will be about 0.60Ohms, which is hardly worse than a C type 60809, but what must be realised is unlike the C type which will be going out on the magnetic trip (so a few tens of ms max), the disconnection with a fault of 360A is taking the full 0.4seconds. Thats an I²t of 51,840 and if you do the adiabatic you get a min conductor size of 1.97mm, so you're fine if its wired in singes with a 2.5mm earth, no so if it is T/E with 1.5mm and these devcies were around in the days of 2.5/1 (although use of T/E in commercial at the time was less prevelent than today, but it still did happen).
The other edge is quite sharp as well, there does not seem to be any enerergy limiting going on, if you look at the bottom section, the I²t goes up in line with the square of the current, which would be expected (without any limiting) as t for magnetic operation is constant, so working backwards this time, the max allowed I²t for a 1.5mm cpc in a T/E is 29,756 so that works out that at just over 1700A PEFC that the magnetic part won't properly protect the 1.5mm cpc, so you don't want the Zdb any lower than 0.13 really, and a zs of 0.6 at the far end already means its failed there, so maybe 0.46 is maybe the highest zs you'd want to so( a guess, you'd have to graph it to find out where the thermal part atually fails the cable)
So while writing 0.60ohms for your 30A loadmaster is the correct thing to do, you have to do more than just check your value against that figure as the let through energys were a lot more brutal than even re-wireable fuses were
... I've gone on a bit haven't I....
