Perhaps that is the issue... Here are the requested pages:
It is explained in 543.1.1, perform the calculation in 543.1.2 OR use the table 54.3
Discuss Twin and Earth CPC in the UK Electrical Forum area at ElectriciansForums.net
Perhaps that is the issue... Here are the requested pages:
Germany and other European country's were well ahead of us in adopting MCB and RCDs
We are now playing catch up, Europe is now writing the standards.
It is not just the "will it or won't it" function of the earth pin or side strip, etc, but generally they have non-polarised plug so you can swap N & L, and also many EU supplies are TT so needed the RCD incomer at least for any chance for acceptable fault clearance.Yes, but I'd argue thats because over their hodge podge of plugs which do not always mate with an earth.
I don't think TN-C-S is any more likely to break down than TN-S. I it just the consequences that are more serious!They’ve used PME to save money on cable but ultimately it will backfire as the network breaks down and the money they supposedly saved , is spent on repairs
If they stuck to TNS then the open PEN consequences never exist.I don't think TN-C-S is any more likely to break down than TN-S. I it just the consequences that are more serious!
It is explained in 543.1.1, perform the calculation in 543.1.2 OR use the table 54.3
It is not just the "will it or won't it" function of the earth pin or side strip, etc, but generally they have non-polarised plug so you can swap N & L, and also many EU supplies are TT so needed the RCD incomer at least for any chance for acceptable fault clearance.
But you are also right that the UK (and some other countries) biggest blind spot is using TN-C-S and the resulting risk from a PME fault. As mentioned before by @davesparks what they should have done is insist on every new property on PME having its own earth rod(s) as well. Sure one rod is not going to help much with a PME fault, but having many, many more rods would be more fault tolerant than a handful of supply rods.
The way I interpret it you must perform a calculation
No it's very clear you must EITHER perform the calculation in 543.1.2 OR select from the table 54.3
it is not ambigus
From an electronic engineering standpoint, TN-C / TN-C-S is fundamentally flawed. You cannot use a conductor to simultaneously establish an equipotential and pass a current, unless it has zero resistance. My opinion is coloured by the fact that I design studio-grade analogue audio electronics as part of the day job, for which the resulting circulating currents and CPC / true earth voltage gradients can be a serious nuisance that would in theory be almost eliminated with pure TN-S
From an electrical standpoint, I can accept that with suitable engineering standards adhered-to rigidly, the additional risk of open PEN faults could be mitigated so as to be an insignificant contributor to the total risk arising from the use of electrical power.
But returning to the subject of CPC size, has anyone here done any practical experiments to satisfy themselves of the validity of the adiabatic limit? I have, years ago, using a very large battery bank, and the results were as expected and unremarkable.
Then how could a reduced size earth in T&E be compliant unless the installer calculates it?
Because it is a standard circuit and it has already been calculated - the whole point of standard circuits is that all the factors have been calculated!
You could calculate it again, but why?
If I have a circuit using class 3 MCB type B up to 16A table B7 in the OSG tells me that is suitable for no less than 1mm^2 up to 3kA
Knowing this - I now calculate it again - Why???
OK, so k=115 the let through from 3kA based on a 16A mcb is 1.98kA, and the trip time is 0.003s (it's well above the inst trip into current limiting)
This works out as SQRT(1980x1980x0.003) / 115 = 0.94mm^2
Isn't that odd, you use the figures provided by the IET as acceptable - and when you calculate it - it actually works out!
Now I go to a different site, and need another circuit using class 3 MCB type B up to 16A table B7 in the OSG tells me that is suitable for no less than 1mm^2 up to 3kA
Do I calculate it again?
In reality few electricians have to use the adiabatic equation (though they should know how to) as the IET's On Site Guide has some useful tables that incorporate the information for circuit design. For example this table is for BS88 fuses and shows the maximum Zs values for different CPC sizes:
View attachment 58753
For example, if you have 10mm T&E cable with a 4mm CPC used as a sub-main feed so you could allow 5s disconnection, you might have a 63A fuse for short circuit protection only, and then the downstream DB can use a mix of MCBs up to 32A in order to provide overload protection with a reasonable chance of selectivity. Looking at the above table you see you max measured Zs is 0.49 ohms, so your final test at the nice new DB would be to confirm this is met.
Also you see the value is 0.62 ohms in all the larger CPC sizes - they are time-limited for the fuse action, where as at 4mm it is adiabatically limited (hence lower Zs for a shorter fault disconnection time).
OK, so k=115 the let through from 3kA based on a 16A mcb is 1.98kA, and the trip time is 0.003s (it's well above the inst trip into current limiting)
On a side note. Instantaneous in current liming like a fuse? As I understand it a fuse begins to melt as soon as it gets hot, while a solenoid must saturate, pull in, and then wait to unlatch with an arc which takes time to extinguish.
I know that RK low peak fuses tend to reduce arc flash to a big degree relative to instantaneous tripping of molded case circuit breakers and power circuit breakers .
Ill take your word for it. That if it means if means loop impedance requirements are met than the CPC will always be greater than what the adiabatic method would calculate out to be when dealing with T&E.
Yes, although the I^2t let through is very much more than a fuse will give; for the very reasons you state
I do not like MCB - fuses are so much better for protecting circuits, coordinating with each other and providing an overall better system.
Unfortunately fuses are inconvieniant - so MCB end up being king!
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Slightly more to it than that, firstly you have to ensure the protection operates within the time limits (which is via selection of correct MCB and the Zs {loop impedance} requirements)
Then, secondly that the CPC is larger than the minimum size given the MCB type, class, and fault level.
So if I have a fault level less than 3kA, a 20A class 3 B type MCB then:
1) The Zs must be less than 1.75ohm (at the time of install - i.e. when cold)
2) The CPC must be bigger than 1.5mm^2
1) can be found from table B6 in the OSG
2) can be found from table B7 in the OSG
In which case standard 2.5mm^2 T&E can be used
Change the fault level to between 3kA and 6kA then again from B6 & B7:
1) The Zs must be less than 1.75ohm (at the time of install - i.e. when cold)
2) The CPC must be bigger than 2.5mm^2
- In this case you can't used standard T&E , and is often the case for industrial sites where the CPC has to be the same size as the live conductors
(of course you would also check for volt drop, cable rating due to installation method etc.)
Above 6kA fault level then you need the manufacturers data and have to calculate manually (although there are other tables available - just not in the standard stuff)
In this case you can't used standard T&E , and is often the case for industrial sites where the CPC has to be the same size as the live conductors (of course you would also check for volt drop, cable rating due to installation method etc.) Above 6kA fault level then you need the manufacturers data and have to calculate manually (although there are other tables available - just not in the standard stuff)
And thats my point right there- you do indeed have to check. I'm not sure what the max PFC is in UK supplies but in large cities like Manhattan, Brooklyn, Queens and Bronx where underground secondary networks are used 22,000 amps or more at a residential service is not unheard of.
Of course the NEC is silent on this issue- just that 2.08mm2, 3.31mm2, 5.26mm2 circuits must have an equal size CPC.
All equipment is designed not to shock you (I hope!) but the reversible polarity means everything has to be double pole safe.I agree, but I don't think polarity makes much of a difference. My understanding is the EU sockets are designed such reverse polarity will not shock you.
The noise level is usually not a problem. In cases where it is you have a far bigger s*if-show with crappy SMPUSU to worry about...From an electronic engineering standpoint, TN-C / TN-C-S is fundamentally flawed. You cannot use a conductor to simultaneously establish an equipotential and pass a current, unless it has zero resistance. My opinion is coloured by the fact that I design studio-grade analogue audio electronics as part of the day job, for which the resulting circulating currents and CPC / true earth voltage gradients can be a serious nuisance that would in theory be almost eliminated with pure TN-S
It is a low risk, but equally it is among the "single point of failure" risks that we ought not to see in power distribution.From an electrical standpoint, I can accept that with suitable engineering standards adhered-to rigidly, the additional risk of open PEN faults could be mitigated so as to be an insignificant contributor to the total risk arising from the use of electrical power.
This is a factor in design that seems to be overlooked a lot. Fuses are a very simple device, so simple that the behaviour is often not understood well, but when it comes to it they do a much better job of limiting fault energy than MCB or MCCB.I do not like MCB - fuses are so much better for protecting circuits, coordinating with each other and providing an overall better system.
Unfortunately fuses are inconvenient - so MCB end up being king!
Officially the max PFC is given at 16kA but in reality you are very unlikely to see more than 6kA. Also the incoming DNO fuse is probably going to limit it further. For example, a BS88 fuse at 100A rating has a peak fault current below 16kA even at 100kA symmetric RMS value of PFC.And thats my point right there- you do indeed have to check. I'm not sure what the max PFC is in UK supplies but in large cities like Manhattan, Brooklyn, Queens and Bronx where underground secondary networks are used 22,000 amps or more at a residential service is not unheard of.
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