I have been asked a question by a guy who is presently preparing to
undertake a C&G 2391 practical assessment. I can see solutions but not
ideal so I wonder if other minds can suggest something diferent and
The situation is this
100A three phase supply TN-S with Ze of 0.8 ohms by enquiry (actual
measured value 0.76 ohms) The first DB in the chain (DB1) includes a
63A single phase breaker protecting a single phase sub-main supplying
a consumer unit in a flat above the workshop. The sub-main consists of
16mm conductors for both live and neutral and a 16mm earth conductor
so Zdb at the consumer unit is measured as 0.77 ohms. The circuits
protected by the breakers within the consumer unit all comply with the
regs relating to max earth loop impedance taking account of rule of
The problem is that the max impedance for a 63A type B MCB is
tabulated at 0.71 ohms and rule of thumb takes this down to less than
0.6 ohms required.
Considering the requirements as part of the test results of DB1 the
circuit does not comply with requirements and the schedule of test
results and notes to the intermediate inspection report reflect the
If this occurred in real life, rectification would be required. His
question to me was how best to modify the circuit to comply.
As I see it there is no possibility of gettinig the Zs down to 0.6
ohms as Ze is higher and I cannot think of any way to get a negative
value of R1 and R2 for the circuit conductors. An easy solution is to
provide RCD protection for this circuit but the RCD will then provide
whole house (flat in this case) cover and leave the occupants
vulnerable to losing lights and everything else in the event of an
earth leak on one of the other circuits in the consumer unit. This is
generally considered unsatisfactory by present day standards.
Using a 100mA time delayed RCD and 30mA quick acting RCD or RCBO to
serve sockets in the consumer unit would probably be satisfactory but
can anyone suggest an easier way which woukd comply with the Regs?
My copy of BS 7671 says 0.76 ohms for a 63 A Type B MCB to EN 60898.
That would seem to be based on Uoc = 240 V and 315 A required (5 * In).
Nevertheless this won't help since you're still looking for a value
less than the measured Zdb. This is clearly a contrived example of an
Another point is that the rule-of-thumb method would be unduly
pessimistic here since almost all of the Zdb is due to Ze. You are
allowed to assume that Ze is constant and apply the temperature
correction factor (F) to the R1+R2 value of the circuit conductors -
this is the basis of the formula
Zs(test) <= (1/F) * (Zs + Ze*(F-1))
given in Appendix 2 of GN3 (p116). But again this is academic in the
A 100 mA or preferably a 300 or even 500 mA time-delayed RCD would be
quite acceptable, IMHO, and surely would be the most cost-effective
solution. Or, as Colin said, get the service upgraded to PME. Driving
earth rods in an attempt to reduce the Ze on a TN system will be utterly
fruitless, unless there is something seriously wrong with the
distributor's network (which we know is not the case since Ze is within
the declared 0.8 ohm value).
Other options might be to reduce the MCB rating from 63 to 50 A, but
that would depend on the assessed MD of the flat, about which we have no
information, or to select a different device to protect the sub-main.
A 60 A BS 3036 fuse is tabulated at 1.17 ohm for 5 s disconnection -
but re-wireable fuses are rather out of fashion these days! An
adjustable MCCB might provide another option - expensive though. Note
reg. 413-02-13 if the disconnection time at the sub-DB (i.e. the flat's
CU) exceeds 0.4 s.
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