Shock overcurrent ignored in recommendations?

[jimny]

It is my reading that doing impedance tests we like a nice low resistance in theory overall and especially for earth , both Ze and R1 or both R1 and/or R2.
This makes a high potential fault current Uo/Ze, way above the allowed fuse sizes for a light circuit or many power too. So do wires really not manage to carry off that potential if they are 1mm or 1.5mm or 2.5mm?
This is my newbie question. Thanks. Held ECS basic card for a term but didn't get any training. Still very keen however to learn how to practice the installation of electricals trade. Gardener and labourer.
Question a bit fuzzy perhaps but the adabiatic equation jumps to using the result of this for finding a cable size. Really you want the resistance to have a suitable protective device for operation yet have gone on to assume the cable will not carry the impedance current or should have gone big csa/big fuse, or the earth fault has to choose between you and the cpc, the neutral already being broken but in a teeny little wire csa, no?

 

[telectrix]

i think what you're getting at is whether the cable will withstand the fault current. take 1.0mm cable. it's rated at 9A ( less if reference method demands), but generally is on a 6A fuse/breaker. the disconnection time is 0.4 secs, which means that the cable only has to pass the fault current for a max. of 0.4 secs., so should withstand a 5x fuse rating for that length of time without detriment. bear in mind that the larger the fault current, the shorter is the duration.

 

[jimny]

Well what I mean is that if Zs is less than 1ohm it is making current available to fault over 230A based on 230V. This seems to require separate reading of manufacturers guidelines for respective fuses as overcurrent in the regs but is simply turned into a result when using the adabiatic equation. Seeing 230A a large cable should be chosen to get to tabulated current (It) after factors so I don't completely see how to ignore the >230A figure.
Doesn't that fault size require larger cable in order for the fuse or RCB to operate fast. Especially when the fault is a wet person holding a metal earthing object. A possible >230A , albeit for a short time for a breaker or with a fuse somewhat longer. This needs a larger wire though still a smaller fuse as this current is unwanted, fast.
I understand this is why we have fuses and RCD's but am unsure that the csa of now the earth is being considered for the impedance calculations.
Is it not?

 

[telectrix]

the adiabatic equation will give you a minimum size for a cpc once you have the fault current value and can get the disconnection time from the current/time graphs in app.3 of BS7671.

 

[jimny]

Thanks. The fault current would be very high though assuming a low resistance and it seems your first reply about small sizes of cables being rated suggests they are not suited to carry fault current. The fault could occur though or simply move on from I = V/R here. It is not Uo/Zs because of a small csa?

 

[jimny]

Not to say that I haven't grasped that even 6A at 230V isn't a heck of alot of energy.

 

[Wilko]

What Tel said
This seems an interesting subject, so putting my student hat on :
434.5.2 has an equation that seems relevant. It gives a guide as to how much current will make a given size conductor go from 70 deg to 160deg (must be upper temp pvc can go for an instant without damage I guess). I put in 1mm2, used k=115 from Table 43.1 and I got 181A for 400ms, 363A for 100ms and a whopping 1,624A for 5ms. Now, the Type B 6A light cct mcb fires off immediately if current is >30A, which from looking at Fig 3A4 means 100ms. Then I looked at Wylex tech info that's online and it says their mcb will clear a hard fault in 5ms or less. So if the prospective fault current of the lighting cct is less than 1600A I'm thinking that 1mm2 will survive a short cct intact with that mcb (and the mcb will cope with 6KA). I'd be interested if anyone can comment please, as it's not something I've thought about before and it may well be a load of trainee rubbish. This is the link to the Wylex data sheet.
https://www.tlc-direct.co.uk/Technical/DataSheets/Wylex/WylexTech.pdf

 

[jimny]

What Tel said
This seems an interesting subject, so putting my student hat on :
434.5.2 has an equation that seems relevant. It gives a guide as to how much current will make a given size conductor go from 70 deg to 160deg (must be upper temp pvc can go for an instant without damage I guess). I put in 1mm2, used k=115 from Table 43.1 and I got 181A for 400ms, 363A for 100ms and a whopping 1,624A for 5ms. Now, the Type B 6A light cct mcb fires off immediately if current is >30A, which from looking at Fig 3A4 means 100ms. Then I looked at Wylex tech info that's online and it says their mcb will clear a hard fault in 5ms or less. So if the prospective fault current of the lighting cct is less than 1600A I'm thinking that 1mm2 will survive a short cct intact with that mcb (and the mcb will cope with 6KA). I'd be interested if anyone can comment please, as it's not something I've thought about before and it may well be a load of trainee rubbish. This is the link to the Wylex data sheet.
https://www.tlc-direct.co.uk/Technical/DataSheets/Wylex/WylexTech.pdf

Wow. That's interesting, that to make t low the equation either needs lower csa or higher current. (K relatively stays within a factor of less than 2)
High current for a speedier shut off is obviously a good thing.
To be able to get to that high current is my concern. It seems that if that is a possible current it should by definition be the current in the equations but I think I am getting it now. If it is above the service operating temperature then whether it is 5 seconds or 5 minutes it is not suitable and so who needs an equation for the wrong wire.

 

[jimny]

Also it will be overheating and causing more resistance very rapidly or at least changing the current transporting ability of said wire.

 

[Wilko]

Hey - site is back up - good job!
Took the opportunity to read the Wylex tech info more and used their graphed I2t values and the adiabatic eqn 543.1.4 ... With Wylex mcb rated at 6KA in a cct with prospective fault current of 6KA (or less) then 6A and 16A mcb are safe with 1mm2 CPC and 20A mcb is safe with 1.5mm2. My calc was CPC required to be 0.8, 1 and 1.4 mm2 respectively. Sounds like someone designed it correctly, or I've nicely mashed the calc

 

[Marvo]

The fault current that will flow during a short circuit will depend on the impedance reading at the point of the fault, not the Ze. You've got to take the resistance of the circuit cabling into account.

Also, as already stated it's better to have a large fault current that gets cleared quickly by the protective device. Low fault currents and long disconnect times cause fires.