Needing to meet mcb zs with ln loop.

The RCD is only providing the ADS for earth faults - it won't "see" a short circuit fault


If the L-N loop is so long, then you also have potentially limited or no short circuit protection


The MCB "Zs" is simply a means of stating what the minimum earth fault and short circuit current needs to be for a stated time


Take note however, that for short circuits, there is no "minimum time" other than that dictated by the cable not bursting into flames - an adiabatic check essentially


Regards


OMS

mcb max zs

balderdash and piffle. The limit for L_N loop is set by other things.

There are two restrictions on L-N loop - firstly  voltage drop at full load must be acceptable - and if VD is say a stonking 10% , then you will only just fire a C type in the instant trip part of the curve on an LN fault. 

It  is quite hard to design a circuit that works where the disconnection time is more than 5 seconds. You may not care unless  the case below also applies.

.

Second case is for  the adiabatic as above - this only applies if the rating of the cable is thinner than the In of breaker that protects is, i.e. for cases where overload is unlikely or there is  overload protection at the load end.

So the danger case is a 32A breaker protecting a 100m run of 1mm2 with a single socket at the end of it.

mapj1:

So the danger case is a 32A breaker protecting a 100m run of 1mm2 with a single socket at the end of it.


 

 

Or a motor, to give a pretty frequent example where OL is at the load end, and the protection needs to deal with inrush current and still provide protection from short circuit to the supply cable


Regards


OMS




 

Is the question being asked regarding a installation supplied from a onsite generator?

https://communities.theiet.org/discussions/viewtopic/1037/23993


Andy

Hi Andy, no the generator one is a different installation, thanks.

Hi Andy, no the generator one is a different installation, thanks.

When testing a RCD protected TT installation where the circuit Zs tested live to CPC could be over a hundred ohms and still satisfactory I may do a live to neutral loop test and if the test result is more than the max MCB Zs I would consider further investigation is required.


When undertaking minor works on an existing circuit you need to be able to quickly assess the condition of the existing circuit and  completely stripping it down to undertake extensive testing is generally not a real possibility.


Andy

A live neutral loop test is the precursor to obtaining the prospective short circuit current test result. On a Megger LT320 loop tester you do the live neutral loop test then when the result is displayed you press the PSSC button for it to calculate and display the PSSC.


So if you obtain the PSSC using such a tester based on a live neutral loop test can you compare the test result with the amperage that is given in BS7671 that is required to operate the circuit protective device within the required time?


Andy 

but the required clearing time for an L-N fault is much longer than for an L-E fault, where we would have to worry about touch voltages, and insist on ADS in a fraction of a heartbeat. There is no formal time limit in BS7671, but If you are thinking it ought to be no more than a few seconds however, then you can deduce a new, much higher level of L-N loop impedance that is the upper acceptable  limit.

Working with the typical B type MCBs found in most domestic and many other types of electrical installations may not call for precise testing .

Referring to Fig3A4 in BS7671 gives operating times for type B MCBs across a range of prospective currents.

There is also a table giving the current for time 0.1 sec to 5 secs required to operate the MCB. The table is really a prompt to remind you that the current for a B type MCB is five times it's rating .

So for example if you have a shower circuit protected by a B40 MCB so long as the prospective short circuit current is more than 200 amps measured by a live neutral loop test and the PSSC calculated from the result it's job done isn't it?

Apart from ticking the box to say the voltage drop has been verified. ?