CPC size help

[RodMan50]

Your welcome, be best you learnt the how tos and why fors

Shame you are not open to discussion :ack2:

 

[Deleted member 26818]

There is nothing in BS7671 or ESQCR prohibiting the exportation of a PME earth.
There are problems when the location the earth is being exported to has extraneous-condutive-parts, in sofar as an appropriately sized bonding conductor will be required.
Often this prohibits the exportation due to cost.

 

[pencilpusher]

There is nothing in BS7671 or ESQCR prohibiting the exportation of a PME earth.
There are problems when the location the earth is being exported to has extraneous-condutive-parts, in sofar as an appropriately sized bonding conductor will be required.
Often this prohibits the exportation due to cost.

Precisley.....I have already pointed out there are NO extraneous conductive parts in the garage.

 

[Engineer54]

Are you really going to export a PME earth ?

Are you another of these scaremongers that that think PME supplies can't be extended??

Well that depends on what side of the fence you sit on

Plenty of export this and export that guides here, there and at the ESC etc

Shame really :ack2:

I take it your a TT man then, ....going by your handle here??

 

[D Skelton]

Are you really going to export a PME earth ?

Yeah, I mean why not just whack an earth rod in instead and introduce an incomprehensibly high Zs!

 

[Deleted member 26818]

I have an extension lead, that uses 3core flex.
Should I disconnect the earth each time I use it outside, so as I don't export my TN-C-S earth?

 

[pencilpusher]

I think you should disconnect the earth and put a temp rod in and earth your lead up to that. When you have finished cutting the grass, rip that rod out....!!

 

[Engineer54]

Good Luck

How much experience have you with TNCS,PME and TT and exporting earths and the problems associated with it ?

WOW!! two new doom and gloom scaremongers in a week!!! Both trying to belittle experienced electricians, trying to make out they know better!! Quite amazing just how widespread these Myths extend, along with there perpetuation by these doom and gloom merchants, that think they know better than You!!

 

[pencilpusher]

Very true but that particular poster does come across as an obnox know it all, hence why I sort of blanked him. I am up for a discussion but not with the likes of him. There are still some dinsosaurs out there where there is no chance of extinction!!

I get some great advice on here but there also seem to be some bitter people on here too!

 

[Deleted member 26818]

I must admit, to being a Bitter person.
That Larger is just too bland for my liking.

 

[pencilpusher]

I must admit, to being a Bitter person.
That Larger is just too bland for my liking.

If you say so, but you do try and help others!

 

[pencilpusher]

To determine the CSA required for a CPC, you need to furnish the type and rating of the overcurrent protective device.
If for instance you intend using a 32A type B MCB, the time/current graphs in Appendix 3 of BS7671, indicates that the PEFC required to provide disconnection between 0.1 and 5s is only 160A.
Your calculated value of 800A will be more than sufficient to to allow the device to operate, and according to the adiabatic equation, will require a minimum CPC CSA of 0.44mm².

I am back thinking about this example again after reading similar threads this week. In another thread http://www.electriciansforums.net/e...bonding-post453351.html?highlight=#post453351

I asked a question in that thread and showed an example of 0.8 kA PEFC at a scond DB but was advised in the thread that I should consider the CPD fitted which I understood and the the appendix 3 graphs to be used. Fine but in this thread I asked a question and I think i was given a different answer. It no doubt my misunderstanding but I need clarification please.

When using the adiabatic equation, what fault current do I use. The actual measured fault current ie 0.8kA in linked thread or the fault current required to trip the CPD which would be much lower. I seem to have got two different answers??

I hope I have made this slightly clearer than mud but this adiabatic game is confusing me.

Thanks

PP

 

[Guest123]

When using the adiabatic equation, what fault current do I use. The actual measured fault current ie 0.8kA in linked thread or the fault current required to trip the CPD which would be much lower

You would use your actual measured fault current if you have it available which would be 800A in this case. What the OCPD requires doesn't come into it.

 

[pencilpusher]

You would use your actual measured fault current if you have it available which would be 800A in this case. What the OCPD requires doesn't come into it.

Lenny thanks but that does not fit in with the answer i got earlier in this thread and that is what is confusing me. I refer to the T&E I had installed that is joined to the SWA. I had 800 amps and thought my min CPC should be 5mm2 or so. I received an answer stating it should be 0.44mm2 ish. Which one is right?

PP

 

[Guest123]

2.1 Calculation in accordance with Regulation 543.1.3 Calculation uses one of two methods permitted by Regulation 543.1.3: [TABLE="class: listitem_table"] [TR] [TD="class: listitem"]•​

[TD][/TD]
[TD="class: listitemtext"] the adiabatic equation given in Regulation 543.1.3, and[/TD]
​

[TR]
[TD="class: listitem, width: 10"]•[/TD]
[TD="width: 10"][/TD]
[TD="class: listitemtext"] the non-adiabatic method given in BS 7454. (This method is not covered in this topic).[/TD]
[/TR]
​

The adiabatic equation is reproduced as follows, together with the explanation of the symbols used in it, as given in Regulation 543.1.3.
[TABLE="class: blockquote_upright, width: 2"]
[TR]
[TD]

Adiabatic equation​

Where:
[TABLE="class: where"]
[TR]
[TD][/TD]
[TD="class: term"]S[/TD]
[TD="class: term"]is the nominal csa of the protective conductor in mm2[/TD]
[/TR]
[TR]
[TD="width: 100"][/TD]
[TD="class: term, width: 100"]I[/TD]
[TD="class: term"]is the value in amperes (rms. for a.c.) of the fault current for a fault of negligible impedance, which can flow through the associated protective device, due account being taken of the current limiting effect of the circuit impedances and the limiting capability (I2t) of that protective device[/TD]
[/TR]
[TR]
[TD="width: 100"][/TD]
[TD="class: term, width: 100"]t[/TD]
[TD="class: term"]is the operating time of the disconnecting device in seconds corresponding to the fault currentI in amperes[/TD]
[/TR]
[TR]
[TD="width: 100"][/TD]
[TD="class: term, width: 100"]k[/TD]
[TD="class: term"]is a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor material, and the appropriate initial and final temperatures of the conductors.[/TD]
[/TR]
[/TABLE]
[/TD]
[/TR]
[/TABLE]
​

[/TD]
[/TR]
[TR]
[TD][/TD]
[TD="align: center"][/TD]
[/TR]
[TR]
[TD][/TD]
[TD]

The adiabatic equation is based on the assumption that the duration of the earth fault current is so short that none of the heat energy produced in the protective conductor escapes before the protective device operates. The assumption, though never true, is reasonably accurate providing the operating time of the disconnecting device does not exceed 5 s, as is normally required by BS 7671.
The csa of a protective conductor, where calculated using the adiabatic equation, must be not less than the required value (S) given by that equation.

2.1.1 Values of I and t or I2t for use in the adiabatic equation
Source of values of I and t
The value of prospective earth fault current (I) to be used in the adiabatic equation is normally determined by calculation, due to the circuit generally not yet having been constructed. The value of operating time (t) for the disconnecting device, for use in the equation, is that corresponding to the value of I, and is ascertained from the time/current characteristic for the device (such as given in Appendix 3 of BS 7671).

Source of values of I2t (for situations where the use of separate values of I2 and t is impracticable or unreliable)
The use of separate values of I2 and t in the adiabatic equation is impracticable where the value of I is so high that a corresponding value of t is not shown in the time/current characteristic for the disconnecting device. In such circumstances, a value of energy let-through (I2t) should be substituted into the adiabatic equation. The I2t value should be obtained from the energy let-through characteristic for the disconnecting device, published by the manufacturer, which is normally in the form of a line or curve giving maximum values of I2t as a function of prospective current under stated operating conditions.
The use of a value of energy let-through (I2t) in the adiabatic equation may also be necessary in the following circumstances, in which the use of separate values of I2 and t is an unreliable basis for calculation:
[TABLE="class: listitem_table"]
[TR]
[TD="class: listitem"]•[/TD]
[TD][/TD]
[TD="class: listitemtext"]short operating times (less than 0.1 s) where asymmetry of current is significant, such as for a protective device close to the output terminals of a generator or transformer[/TD]
[/TR]
[TR]
[TD="class: listitem, width: 10"]•[/TD]
[TD="width: 10"][/TD]
[TD="class: listitemtext"] where the protective device is a current limiting circuit-breaker or fuse and the prospective earth fault current is of such magnitude that the device will ‘cut off’ or limit the current during fault conditions (generally in less than half a cycle of the alternating current waveform).[/TD]
[/TR]
[/TABLE]


Assumed position of earth fault
Where the disconnecting device is a fuse, the appropriate values of I and t, or I2t to be used in the adiabatic equation are normally those corresponding to an earth fault occurring at the point of the circuit concerned which is electrically most

​

[/TD]
[/TR]
[TR]
[TD][/TD]
[TD="align: center"][/TD]
[/TR]
[TR]
[TD][/TD]
[TD]

remote from the supply. This is because the characteristics of fuses of the types considered in BS 7671 are generally such that the energy let-through (I2t) is greater the lower the fault current.
The same does not always apply for circuit-breakers, however. It will often be found, from the energy let-through (I2t) characteristic of a circuit-breaker, that for fault currents exceeding a certain value (normally somewhat higher than that causing operation of the device within 0.1 s), the energy let-through (I2t) is greater the higher the fault current. As a general rule, where values of prospective earth fault current exceeding about 1 kA occur anywhere in the circuit, the energy let-through characteristic of the device, obtained from the manufacturer, should be consulted. From this characteristic, the maximum value of I2t for the range of values of prospective earth fault current occurring throughout the circuit should be ascertained, and should be substituted into the adiabatic equation.
​

2.1.2 Value of k for use in the adiabatic equation
Values of k for protective conductors in various use or service, for use in the adiabatic equation, may be obtained from Tables 54.2, 54.3, 54.4, 54.5 and 54.6 of BS 7671.

2.1.3 Protective conductor common to a number of circuits​

​

Where calculated, the csa of a protective conductor that is common to several circuits is to be based on the most onerous of the values of earth fault current (I) and operating time (t) (or energy let-through (I2t)) encountered in each of the circuits which share the cpc (Regulation 543.1.2).
​

2.1.4 Where the application of the adiabatic equation produces a non-standard conductor size
Where the application of the adiabatic equation produces a non-standard conductor size, a protective conductor of at least the nearest larger standard csa has to be used.
​

2.1.5 Canonical form of adiabatic equation

Finally, it should be mentioned that the adiabatic equation can be expressed in canonical (basic) form, as given below, in which the prospective earth fault current (I) is not squared, because it is brought outside of the square root sign.

​

[/TD]
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