the 'laws' of Ib <= In <= Iz and I2 <= 1.45 Iz re: direct buried cable

This has realised for me that I need to read some more about this 'correction factor' subject  and of how the values in the tables have been realised.

For all in the world I would have taken 433.1.203 (compliance with In <= 0.9 Iz to meet 433.1.1iii) as a requirement  in relation to my OP example.  It seems that is not the case as the tabulated (It) value already has it built in so to speak, so all that is needed is to ensure Ib <= In <= Iz (and in my OP example it was so). Are there any dissenters ? :-)

thank you

psychicwarrior said:

Are there any dissenters ? :-)

Well, me ... I'm not 100 % sure it's clear.

Have a look at the last para in Section 4 of Appendix 4:

The tabulated current-carrying capacities for cables direct in ground or in ducts in the ground, given in this appendix, are based an ambient temperature of 20 °C. The factor of 1.45 that is applied in Regulation 433.1.1 when considering overload protection assumes that the tabulated current-carrying capacities are based on an ambient temperature of 30 °C. To achieve the same degree of overload protection where a cable is “in a duct in the ground” or “buried direct” as compared with other installation methods a rating factor of 0.9 is applied as a multiplier to the tabulated current-carrying capacity.

So, can I ignore it, because the 1.45 factor in 433.1.1 (as stated in 433.1.201) is ignored, unless I'm using a BS 3036 fuse?

There are no examples provided in the IET's publications (as far as I'm aware) to definitely hang this off, which I can point to off the top of my head.

I'm sure, to be on the safe side, the 0.9 is usually applied by calculation software ... but I don't think BS 7671 makes itself 100 % clear in this regard.

I will now stand back a little while and see what else comes back.

:-)

well the CCC of RefC is 67  -   60 * 1/0.9  is about that, so is there something in that to imply the 0.9 is already captured in the It of RefD ... or am I just waffling ....

I wouldn't really consider it like that. Table 4D4A clearly says ground temp is 20 deg C and assumed conductor temp 70 deg C.

BUT

There is still nothing to tell me whether I can ignore C_c if and when C_f = 1 (in those simple terms).

In reality, I think the answer is NO, you can't .... but I can't think of where I could point you to tell you 100 % definitely for sure in a very clear manner.

The quoted para you presented from App4 is not exactly clear as you point out .

It could be intepreted as suggesting that the tabulated figure has had the 0.9 factor applied such that the CCC is 60A  to cater for the [ground] ambient 20DegC  (and not the [ambient] 30DegC elsewhere).

Or it could be suggesting that a further 0.9 is required to be applied to the tabulated 60A - but why not just have the correct tab value in the table in the first place.  One wonders what the CCC is for an ambient at 30DegC then !

I feel (very scientific) that you are right that the correction is already in the 60A.

based on the OCPD value to provide overload protection:

It >= 63 / (0.9 * 1.1 * 1.04)

It >= 61.1  , or  55.0 without the 0.9 !   10mm2@ It of 60A is no good for the former but good for the latter

The Iz of the cable is 60A  when buried @ ambient of 20DegC  and that's that.  It can carry more depending on other conditions  e.g. if it's closer to surface, if the soil is at 1.2 not 2.5 and the notes (7.1 note 3 Appendix 4) also suggest that the CCC is approx. 10% higher than those given in the table (hence the 1.1 factor - not sure if that applies to if in ducting/conduit when in the ground though). The 0.9 seems applicable only when considering the OCPD to Iz.

Therefore Iz = 68 = 60 * 1.1 * 1.04  is >=  It of 61  (from above)

Summary:

if you were suggesting the Cc 0.9 is not applicable when working out Iz from the tabulated value, I think you are correct.  It is required if determining the CCC required in relation to the OCPD for proving compliance with  I2 <= 1.45 Iz (for the stated devices).

What do you think ?

Also but as an aside, what about that 7.1 Note 3 (10%) when in a duct in the ground (still RefD), is it applicable ?

psychicwarrior said:

Also but as an aside, what about that 7.1 Note 3 (10%) when in a duct in the ground (still RefD), is it applicable ?

Yes, the values in BS 7671 are very conservative, but it does depend on the soil thermal resistivity.

psychicwarrior said:

What do you think ?

In all honesty, I really don't know without giving it some serious thought. Now, the 0.9 is usually applied, but "just because we've been told to" isn't the same as "BS 7671 clearly says so" as we've just established.

Considering other factors, if, as usual, the cable is rarely, if ever, anywhere near fully loaded, the starting conductor temp for the overload is nowhere near 70 deg C, which means the cable will take time to heat up just as the OCPD ... and it's unlikely to be a big issue if you didn't apply the 0.9 in those cases, whether "needed" or not. It's only a problem if the starting temp > 60 deg C really (makes up the difference between the 20 and 30 deg that's being discussed in the last para of item 4 of Appendix 4).

In any case...

I'm convinced that when working out Iz from tabulatedfor RefMethD, that 0.9 isnt required   - also because if overload prot. was ommitted then it doesnt matter at all other than PSSC protection being satisfied (as in Ib <= Iz).

So based on the example I used (and repeating somewhat) and inc. the Note 3 10% mentioned:

For the Iz = It * (correction factors) ... the Cc 0.9 is not needed and can be taken as 1.

i.e.   Iz = It * Cd * Cc * 1.1  = 60 * 1.04 * 1.0 * 1.1 = 68    (no mention of In of course)

But for overload protection given the tabulated current is based on ambient at 20DegC, the check needs to be made against the OCPD being used, in that compliance  with I2 <= 1.45Iz is achieved using  Iz >= In / (correction factors).

i.e. I2 * 1.45Iz compliance can be taken as met (for the specified devices) if  Iz >= In / (Cd * Cc * 1.1)    =   68 >= 63 / (1.04 * 0.9 * 1.1) ... 68 >= 61.2 [tick]

and lastly  Ib <= In <= Iz   =  32 <= 63 <= 68  [tick]

I do believe you were right with your original comment about the 0.9, as I interpretted it and applied it above.   perhaps others will come along and demonstrate otherwise.

Thank you for your comments.

I'm going to have to read this in details again (I'm not yet sure where * 1.1 * 1.04 comes from, I was expecting just 1.03 for 0.5m deep direct burial, and possibly 0.9 for overload protection of "20 degree ambient " buried cables).

I did think though that the 0.9 factor for overload protection of buried cables wasn't included in the tabulated numbers though - that way if you didn't need overload protection you could use the whole of It, if you did need overload protection you're limited to It * 0.9.

I can see confusion over whether that 0.9 is included in Iz - the wording of 433.1.203 suggests it isn't - which is a bit odd since all the other correction factors usually are.

Just as an aside, if Ib is only 32A, why not go for the 32A/40A/45A OPD and design out the problem?

   - Andy.

Andy, the 1.04 was from a table in another book about depth and the 1.1  I mentioned, is a comment that the figures tabled are conservative and in reality are perhaps 10% higher for the tabulated conditions  (ref clause 7.1 app4 ?).

I believe the 0.9 is related to the OCPD  check for overload protection of buried cables - in relation to I2 <= 1.45 Iz or I2 >= 1.45 In and because of the tab values being based buried ambient 20DegC (not just ambient 30 as with others) when it comes to that check,  the equation becomes I2 <=1.3Iz or I2>=1.3In  (or so I read somewhere)  and if working out Iz  from tabulated  - as in Iz = It * (correction factors) -  then that is not concerned with OCDP and only what the cable can carry given the conditions.   Once overload is present then the 0.9 seems to come into it ...  but its all clear as mud now to me  !     I started out with it 'in' then took it out based on further discussion.

however I have read elsewhere in 7671 that 0.9 has to be applied to tabulated values if overload protection is being employed.     

yes of course the OCPD could be reduced for the example Ib figure -  I was just exploring as a side exercise issue, getting as close to selectivity between a fuse and a possible MCB.   equally perhaps one might consider that overload prot is not needed and the problem goes away too :-)

psychicwarrior said:

I'm convinced that when working out Iz from tabulatedfor RefMethD, that 0.9 isnt required

It would be required for Ref Method C, but I think I'm coming round to the fact that it is (or ought to be) already included in Table 4D4A for Ref Method D (if not, it wouldn't make much sense including the Ref Method column).