gkenyon:

°Zoomup:

Zoomup:

Without doing any calculations and just using previous "experience knowledge" I venture to say that the proposed installation as described will comply.


But I may be concerned about Volt drop. What is the design load and installation method?


Z.

CORRECTION. Now, after doing some quick sums I now believe that the disconnection time will be nearer to 10 seconds with an earth fault of negligible impedance at the far end of the S.W.A. That just shows us what guessing can achieve.......bad results. The C.P.C. needs to be bigger, supplemented with an additional copper C.P.C. or an R.C.D. installed at the origin of the circuit.


Z.

 

How so Z?


A B16 as Max Z_s of 2.73 Ω for 0.4 s disconnection

Less Z_e of 0.35 Ω leaves 2.38 Ω

So, using even the over-excessive final temperature resistances leaves over 90 m to achieve disconnection time of 0.4 s (although for Type B mcb this is the same as 5 s) ???

The limiting factor as you initially said would be volt-drop at 45 m (if it's a final circuit conductor, less if you want to drop short circuits off the other end).

 

gkenyon:

Legh Richardson:


Here's a R1+R2 table used to calculate the earth loop impedance using SWA armour for R2, (Zs), You can then calculate the prospective short circuit current and hence the adiabatic calculation.


Legh

Hang on - this isn't the correct method.


(b) If you're using a circuit breaker (mcb, RCBO, mccb) you cannot use this method, you must use the let-through energy from either the product standard, or let-through energy provided by the manufacturer, and simply compare against k²S² - again, no need to calculate the fault current.


See Chapter 8 of Electrical Installation Design Guide for a complete explanation.

 

gkenyon:

Nope.


What are you getting at?


This particular example hasn't been stated as low fault level, so we must assume 16 kA (and 0.35 omega Ze) - of course could be higher fault level if larger supply than the standard 100 A.


As I stated, it's not the fault level at the end of the cable that's important, it's the fault level at the start of the cable.


However, no para in EIDG still says the same thing.


I'm sure someone will point out that "in many houses the fault level is below 3 kA" - well, it's not in mine, which is often over 6 kA, and I'm over half way down our street from the substation.

Farmboy:

What I'm getting at is asking your advice.

 

Specifically, I'm asking whether Zs can still be used to determine the level of fault current to use in the adiabatic equation, as was suggested in section 8.4.2 of the EIDG (2015), for low levels of fault current (defined in that edn as being below 3kA), or whether that method has now been removed from the 18th edn. If it has, so be it, and it's a case of using the energy let-through data from the manufacturer, or the BS. I don't have the book yet, so that's all I was asking: has it been removed?


If it has been removed, that begs the question then of what protective device fault level should be used if a measured value indicates a low fault level. For example, if a 100A single phase domestic supply has measured loop impedance values at the origin of 1200amps L-N and L-E (just examples), and cb's to BS EN 60898 are used, what device kA fault level should be used, to then determine the respective kA2S value for that fault level to use in the adiabatic equation (would it be minimum 6kA fault level, to comply with the 16kA rating of CU's to BS EN 61439 on a 100A supply)? 


F 

 

gkenyon:

I'm sure someone will point out that "in many houses the fault level is below 3 kA" - well, it's not in mine, which is often over 6 kA, and I'm over half way down our street from the substation.

gkenyon:

Farmboy:

What I'm getting at is asking your advice.

 

Specifically, I'm asking whether Zs can still be used to determine the level of fault current to use in the adiabatic equation, as was suggested in section 8.4.2 of the EIDG (2015), for low levels of fault current (defined in that edn as being below 3kA), or whether that method has now been removed from the 18th edn. If it has, so be it, and it's a case of using the energy let-through data from the manufacturer, or the BS. I don't have the book yet, so that's all I was asking: has it been removed?


If it has been removed, that begs the question then of what protective device fault level should be used if a measured value indicates a low fault level. For example, if a 100A single phase domestic supply has measured loop impedance values at the origin of 1200amps L-N and L-E (just examples), and cb's to BS EN 60898 are used, what device kA fault level should be used, to then determine the respective kA2S value for that fault level to use in the adiabatic equation (would it be minimum 6kA fault level, to comply with the 16kA rating of CU's to BS EN 61439 on a 100A supply)? 


F 

 

The method is still valid, but it does NOT involve finding Z_s of the circuit - see note in (b) below


It's first necessary to determine the fault level to choose which method to use. The largest fault level occurs at the start of the cable, so therefore effectively Ze in a single-DBOl installation, or Zdb (at the DB, FCU, etc.) otherwise


(a) If the fault level is over 3 kA, use the energy let-through of the device per the standard, or the manufacturer's data, as per §8.5 of EIDG.


(b) If the fault level is less than 3 kA, use the method per para 1 of §8.4.2 of EIDG, Table 8.2 is an example of the "answers" for Type D mcb's.
NOTE: The data in Table 8.2 of EIDG is NOT based on the fault current calculated by Z_s of the actual circuit, but based on the current obtained by Figure 3A6 of BS 7671 - or effectively 5I_n for a Type B, 10I_n for a Type C, or 2I_nfor a Type D.
Hence, even with this method, it is not necessary to know R₁+R₂.

 

Chris Pearson:

Graham, now you are confusing me.


I can see the point of manufacturing gear to cope with 6, 10, 16 kVA on the basis that the installation may be literally next door to the transformer (so far I have resisted the temptation to knock on the door of our neighbour across the road with my MFT at the ready) but I am not sure why you mention the maximum (PME) external loop impedance.


The adiabatic tells me that 4 sqmm CPCs will suffice for all circuits given a Ze of 0.2 Ω - I cannot be more than 1150 A. If Ze is anywhere up to 0.35 Ω, the equation holds good.


However, I have assumed that Ze is 0.35 Ω when it comes to the ends of circuits because I need to be satisfied that Zs will be low enough to satisfy the requirements of ADS. If Ze is lower, the conditions are still met.


If I have that wrong, please let me know whilst I still have time to change the design.

• For protection against electric shock, this is the maximum loop impedance (and Z_e of 0.35 Ω)

• For protection against overcurrent, this is not the case, and we need to look at the maximum prospective fault current ... both at the start of the circuit and at the end of it.