Confusion over table 7.1(i) OSG

maybe a good idea to go for 1st principles .

A B breaker at the least sensitive edge of the permitted tolerance  needs to be hit with 5 times its nominal rating to guarantee to instant trip. That would be 200A.

The mains may be as low as 230V- 10% or 207V. So

loop resistance of 207/200 = 1.035 Ohm. 

now neither the OSG nor BS7671 actually work like that, but prefer to give tables so we need to guess what has been done.

The OSG, I think has taken the 0.8 ohms of the external network, and decided that 

1.035 for the whole loop, minus the 0.8 ohms outdoors, leaves  a mere 0.235 ohms for the indoors bit, and asked how much cable is that resistance.

1m of 6mm on the way out ~ 3 milliohms and 1m of 2.5mm core on the way back, 7 milliohms so 10milliohms per metre, and  0.235/0.010 = 23.5 metres

I suspect by rounding the numbers in various places, like the core resistances you can get closer to 23m exactly.

Its a hack as not almost no TNS systems have exactly 0.8 ohms external loop impedance and almost no B type breakers need the full 5 times nominal to fire, so in reality a longer cct is usually fine.

Mike.

It's late, so this might need a check with fresh eyes, but I think your issue is the way the OSG applies the “rule of thumb” to it's tables ie to compensate for cable temperature.

From BS 7671 Table 41.3 last column, we see the maximum ACTUAL Zs permitted for a B40 is 230x0.95/(5x40) = 1.0925Ohm or 1092.5mOhm (which matches the tabulated value in 41.3 of 1.09Ohm).

Taking off 800mOhm for Ze leaves a maximum R1+R2 of 1092.5-800=292.5mOhm.

Table I1 of OSG gives resistance/m of 6/2.5 as 10.49 mOhm/m at 20C. Table I3 then tells us that at full working temperature of 70C, we need to multiply by 1.2.

Which gives 10.49x1.2 = 12.588 mOhm/m

Therefore maximum length of cable = 292.5/12.588 = 23.24m.

Or if using the tabulated value of 1090mOhm -800= 290/12.588=23.04m.

Table B6 of OSG gives the maximum MEASURED impedance (ie at 20C). What they seem to have done is apply the compensation to the whole Zs including the Ze part, so  1.09 x 0.8 = 0.872 which nearly matches the 0.88 in the table, but not quite.

(The reason I was working with more digits was to see if 0.8 x 1.0925 gets it to 0.88. It gets it to 0.874 which is a bit closer, but still rounds down to 0.87 rather than 0.88).

So I can see why the answer is 23m and maybe the missing 0.01 will turn up in the morning ?

mapj1: 

. . . The mains may be as low as 230V- 10% or 207V. . . 

Not quite. The lower limit is -6%, which gives 216.2V at the load side of the meter or isolator (where fitted by the supplier). 

Regards,

Alan. 

 

So I can see why the answer is 23m and maybe the missing 0.01 will turn up in the morning ?

 

Am off to work shortly, but would the missing .01 be accounted for by the  figure for Zs in OSG B6 being tabulated at 10degC not 20degC? See note OSG page 126. Correction factor in table B8

Alan Capon: 
 

mapj1: 

. . . The mains may be as low as 230V- 10% or 207V. . . 

Not quite. The lower limit is -6%, which gives 216.2V at the load side of the meter or isolator (where fitted by the supplier). 

Regards,

Alan. 

BS 7671 takes a different approach again, saying 230V x Cmin = 230V * 0.95 or 218.5V. 

   - Andy.

And herein lies the rub, pick a few corner cases - slow MCB, low mains voltage, hot live cable, so higher resistance,  then wonder should the earth path also be hot - probably in T and E on its own, but not if the earth path is assisted by trunking, plumbing  or conduit perhaps, add a bit of rounding up or down, and you will get a whole slew of similar, but not identical answers.

I'm aware of the 10% volt drop being pessimistic .but it compensates for my resistance figures not being ‘hot’ .

You could plug the loop impedance meter in at the far end after you have built it of course but then its a bit late to change it if the numbers come up a bit short.

The tables are good, so long as you realize that they apply only loosely to any specific  real case.

In many ways the last digit is a bit fruit machine - especially when it gives apparently precise answers like 118m or something.

Mike

hertzal123: 
 

When I look at line 10 (B40 mcb on a 6/2.5 twin and earth,TNS system the max circuit length due to zs is given as 23m(no RCD).Yet when I calc R1 +R2 x 23m I get 10.49x23/1000 = 0.24 ohms.Add ze for TNS to this(0.8) gives 1.04 ohms,whilst table B6 gives a max zs of 0.88 ohms for a B40.Wondered where I,m going wrong.

                                                                     Regards,

                                                                                      Hz

                                                                     

Table B6 in OSG is the maximum measured earth fault loop earth impedance. There is no way of getting this to reconcile with the circuit calculations at all. To properly do the calculation, you should use the Zs values from BS 7671.

The calculation method that is recommended, and used for Table 7.1(i) in OSG, is discussed in the IET's Electrical Installation Design Guide (EIDG). Standard circuits are covered in Appendix B, but reference is made to other sections of the book so you can see where it's coming from if you need further explanation on any particular point.

The calculation used is roughly as follows (in LH column, tables in Red are from BS 7671, black the EIDG):
 

Thanks for all the help,I think I understand the method now.

                                                                                 regards,Hz