Sub-mains and more

[JohnW2]

Ah - the joys of let-through energy and adiabatic equtions are heading your way...

I'm well familiar with adiabatic equations but may need some education as regards 'let-through energy' (which isn't in the index of the regs) - although one can stab a guess at roughly what it's about.

One of the problems in my doing any sums is that I don't really know what fault current to design for.

This may be an example of regulations getting a bit wearysome. I think we all know that if I short together the ends of two 1 metre lengths of 10 mm² cable which are protected by a 60-80 A fuse, the cable won't almost certainly won't come to any harm (apart from arcing damage at the end where we created the short), don't we? I think it's fair to say that we also know that, once the cables are attached to the CU (hopefully attached while the supply is isolated!) and its cover replaced, the chances of a short arising in that 1 metre of cable or within the CU is vanishingly small.

Kind Regards, John.

 

[flameport]

a dead short between the far end of a pair of 10mm² singles about 1 metre long (about 8.8 mΩ combined resistance) ought to result in a current around 26 kA

In reality that will not happen, as the supply impedance will limit the fault current to a much lower value.

This discussion covers let through energy:
http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=20738

 

[JohnW2]

a dead short between the far end of a pair of 10mm² singles about 1 metre long (about 8.8 mΩ combined resistance) ought to result in a current around 26 kA

In reality that will not happen, as the supply impedance will limit the fault current to a much lower value.

That is obvioulsy true, but I don't think it alters the rest of my sentence, which read:

...so I reckon that anything approaching a dead short would blow the service fuse mighty quickly, I doubt appreciably slower than would a 60A fuse.

...so I guess I'm still having some difficult in understanding the thinking behind the requirement to add a second, potentially similar/identical, fuse in series with the service fuse for longer cable lengths. To provide discrimination (which I think we're now meant to call 'selectivity') between the company's fuse and the user's one would presumably require the latter to be smaller in rating than the former by a factor of at least 2 or 3 - which is generally not the situation we are talking about.

This discussion covers let through energy:
http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=20738[/QUOTE]
Thanks. I'll take a look at that.

Kidn Regards, John

 

[westie101]

so I guess I'm still having some difficult in understanding the thinking behind the requirement to add a second, potentially similar/identical, fuse in series with the service fuse for longer cable lengths. To provide discrimination (which I think we're now meant to call 'selectivity') between the company's fuse and the user's one would presumably require the latter to be smaller in rating than the former by a factor of at least 2 or 3 - which is generally not the situation we are talking about.

To reduce the risk of DNO staff having to attend to replace our fuse in the event of it operating (and disconnecting all supplies) for a fault on the customer's wiring

 

[JohnW2]

so I guess I'm still having some difficult in understanding the thinking behind the requirement to add a second, potentially similar/identical, fuse in series with the service fuse for longer cable lengths. To provide discrimination (which I think we're now meant to call 'selectivity') between the company's fuse and the user's one would presumably require the latter to be smaller in rating than the former by a factor of at least 2 or 3 - which is generally not the situation we are talking about.

To reduce the risk of DNO staff having to attend to replace our fuse in the event of it operating (and disconnecting all supplies) for a fault on the customer's wiring

Needless to say, that was my first thought - and hence my point. Given the lack of discrimation/selectivity between the DNO's fuse and a nearby consumer's fuse of similar rating (which will often be the case, particularly if we're just talking of a 'long tails' situation), I don't see the requirement as much of a protection against such call-outs - or am I missing something?

Kind Regards, John.

 

[westie101]

The intent of the DNO fuse is to protect their equipment, OK in a lot of cases this includes the consumer unit tails.

It is not there to protect customer wiring. If it was access would have to legally be given to replace it if it operates. There is enough concern about qualified persons interfering with that fuse never mind unqualified persons!

In reality you are as likely to operate the DNO fuse for a fault on a cooker as for anything else (fact not theory)

As the design and installation is also outside the 17th edition etc. it could not be included in any design calculations

 

[JohnW2]

The intent of the DNO fuse is to protect their equipment, OK in a lot of cases this includes the consumer unit tails.
It is not there to protect customer wiring.

It is the very fact that I understand that which causes me difficulty in understanding the thinking behind the requirement for a second (consumer) fuse after the DNO's meter in the case of long tails. I really must be missing something!

Kind Regards, John

 

[ban-all-sheds]

As westie says, it means that their fuse only has to protect their tails and the ones which would normally go to the CU.

 

[JohnW2]

As westie says, it means that their fuse only has to protect their tails and the ones which would normally go to the CU.

Indeed, but that's what I'm talking about, too - i.e. the main (first) part of my initial posting, not the real-world 'add on' question about my 10 mm² sub-main. It seems that there is a requirement for the consumer to provide additional protection (often a fuse of similar rating to the DNO's one) close to the meter in the case of long tails (even if they are just going to a single CU), and it's the thinking behind that which I still don't really understand.

Kind Regards, John.

 

[ban-all-sheds]

One of the problems in my doing any sums is that I don't really know what fault current to design for.

What's the breaking capacity of your MCBs?

 

[westie101]

To slightly turn this, but it may help, given a blank sheet what would you do?

 

[JohnW2]

This discussion covers let through energy:
http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=20738[/QUOTE]

Ah, thanks - so it's just I²t (as per the "434.5.2 equation"). As the discussion observed, whatever this quanity is, it's not energy! I²Rt would be energy (in Joules, with A, Ω and secs), but It has units of Joules/ Ω - which is a strange animal which I don't think has a recognised name!, other than "energy per unit resistance".

Kind Regards, John.

 

[JohnW2]

To slightly turn this, but it may help, given a blank sheet what would you do?

I think this discussion is getting a bit confused - at least, I am! Are you talking about my main question - namely why DNOs seem to want an additional consumer protection for long tails - or the subsidary question about my very short 10 mm² 'sub-main' to a CU with 2 x 16A MCBs?

Kind Regards, John

 

[westie101]

The main question relating to the need for additional protection

The relative sizes of tails/fuses and which specs should be used is a source of much argument within the industry!

 

[NotHimAgain]

Operating I2t (Joule integral)
Integral of the square of the current over the operating time of the circuit-breaker on a short-circuit.
Sometimes referred to as ‘energy let-through’. When expressed in A2t gives the energy dissipated per ohm and
thus represents the thermal effect on the circuit.

Taken from a BEAMA Technical Bulletin.