# Why don' we use RCD trip times for adiabatic equation

When using adiabatic equation for calculating minimum size of CPC, every example I have seen uses 0.1 second or whatever the disconnect time of the mcb element of the RCBO  or MCB will be.

In a domestic sittuation most circuits are protected by RCD's with a trip time of 40mS with significant fault currents, in this sittuation why don't we use 40mS as T in the adiabatic equation?

Parents
• Quite simply because adiabatic is about protection against overcurrent, and RCCBs, or the RCD element of a combination protective device, cannot provide protection against overcurrent.

GN6 tells us (Section 1.5):

While residual current devices (RCDs) can provide protection against electric shock by automatic disconnection of supply, they do not provide protection against overcurrent. Residual current circuit-breakers (RCCBs) must always be backed up by a separate overcurrent protective device to protect against fault current (and, if required for the particular circuit, overload current). Overcurrent protection may be included in the same device, for example, residual-current circuit-breaker (with overcurrent protection) (RCBO).

This does bring into question how we approach the situation for TT systems. The important factor is that we can't assume the prospective earth fault current is determined by the earth electrode alone (in the way we do for ZS for ADS), because extraneous-conductive-parts, or fortuitous earthing, may well reduce the overall effective earth electrode resistance ... and increase prospective fault current.

In the worst-case, prospective earth fault current could well be the same as L-N prospective fault current (see Section 6.4.3 of GN6), and therefore we ought to consider using the same approach for protection against overcurrent for earth faults in TT system earth faults, as TN system.

• Hi Mike,

Thanks for the response.  I get the difference between the two, but wondered why the IET have chosen to use the term overcurrent in the Guidance Note in relation to a section of 7671 that is associated with overload.

• The regs state the definition for an overload current in part 2 (an overcurrent occurring in a circuit that is electrically sound). Therefore 'overloads' are not faults at all and can only really occur in inductive circuits i.e. motors that have jammed. By definition, a resistive circuit cannot have an overload current as a fault must occur before an overcurrent happens. This is why resistive circuits that are not liable to overload conditions, need only fault protection and as such we can 'pass' circuits that initially seem dangerous (a 3kW convector heater on a 40A MCB wired in 2.5mm 6242Y for instance). As long as Zs and the CPC are sufficient then that circuit complies.

Along with the standard confusion over earthing/bonding, I'd say overload current is the most commonly encountered misunderstanding I come across from time-served electricians in the industry.

• Er, final circuits containing sockets can easily become overloaded without any fault, be it a resistive load or otherwise. Just plug too many heaters in.

• Very true, another viable overload condition. Which is why socket circuits would be subject to protection against overload current as well as fault current. Unless the total number of sockets limits the current flow to below the rating of the cable (i.e. a single 13A socket wired in 2.5mm cable) - I don't see why overload protection would need to be offered in that instance.

• Some of us like fairly close protection  upstream fuses anyway - just in case. The rest of the world is not always as well behaved as it should be...

Mind you that bit of 2.5mm2 can be overrun quite a lot for quite a while before it fails, assuming it was designed for a 20 year life at 70C copper temp and halving life every 8 degree or so... table from the 2004 commentary on the amendment to 6,1,3 , no longer sadly free to download.
Note that temperature rise more or less relate to the square of current - so if 27A gets 2.5mm T and E  to 70C from a 30C ambient (40 degree rise) then twice that is an 80 degree rise (to 110 degrees) and takes root 2 times the current or just under 39 amp...

Mike.

• Therefore 'overloads' are not faults at all and can only really occur in inductive circuits i.e. motors that have jammed. By definition, a resistive circuit cannot have an overload current as a fault must occur before an overcurrent happens.

As well as sockets or other situations were end users can change the rating of the load (e.g. by replacing lamps with higher wattage ones), simple resistive loads can develop faults that have all the characteristics of an overload as far as the fixed wiring is concerned - e.g. shorted turns part way along a heating element (or fault to earth part way along a metal sheathed element, especially where there's no RCD upstream).

- Andy.

• I'm still not clear on this point - The discussion that has followed has focused on the types of overload that are feasible to occur,  but i am still not sure why the regulation that is related to overload is effectively broadened by the GN text to include overcurrent.

Is the guidance from GN6 represented in BS 7671?  I can't see it in there.

• Is there a difference between PRC and XLPE generally?

The table and formulae are still reproduced in the latest version of the commentary, albeit with a hefty disclaimer from the BCA. Sadly IEC 943 does not seem to be available even as an obsolete document but judging by a preview of an Australian standard which remixed it is had some other interesting content... Does anyone know what has replaced it?

*sadly not revised since 17th Am3

• I would guess it's because overload events are, by definition, a subset of overcurrent events, and protection against overload is, where required, provided by an overcurrent protective device.

• Indeed some faults can develop that have the characteristics of an overload current, but by definition these are not overload currents and protection can be omitted under 433.3.1(ii) using the definition in part 2. I've lost count of the number of EICRs I've seen where the testers have failed 1mm lighting circuits that are protected by a 16A MCB.

• Indeed some faults can develop that have the characteristics of an overload current, but by definition these are not overload currents and protection can be omitted under 433.3.1(ii) using the definition in part 2. I've lost count of the number of EICRs I've seen where the testers have failed 1mm lighting circuits that are protected by a 16A MCB.

Children
• But surely standard domestic lighting circuits with standard bayonet / Edison fittings aren't fixed loads... There's no telling what shenanigans users might get up to!

(Try searching for "light socket splitter" for a start, though I've heard tell of adapters to BS1363-ish outlets...)

• I've lost count of the number of EICRs I've seen where the testers have failed 1mm lighting circuits that are protected by a 16A MCB.

It would be interesting to see a general justification for 16A MCBs protecting 1.0mm² conductors for faults - 434.5.2 points us to confirming that I²t ≤ k²S². A PVC insulated conductor typically has k=115 so 1mm² has a withstand (k²S²) of 13,225 A²s. Compare that with BS EN 60898's generic values for energy let-through (I²t) for a B16 of 15,000 A²s at 3kA, rising to 35,000 at 6kA (or 18,000 rising to 42,000 for a C16).

For a B6 (or C6), as In ≤ Iz we can at least use 435.1 to deem it OK (presuming the MCB has the required breaking capacity).

Some MCB manufacturers publish their own lower figures for energy let-though, which may tip the balance in your favour in specific cases of course, but that's a long way from being able to make a blanket statement of suitability.

- Andy.

• Indeed, though a lot depends on the type of wiring. For identically sized conductors, table 54.7 can be used to determine suitability without even needing to do an adiabatic. Would be difficult to state that the circuit wasn't compliant, especially if 433.3.1 was applied.

• For identically sized conductors, table 54.7 can be used to determine suitability without even needing to do an adiabatic.

Only for c.p.c.s - you still have the problem of proving that a 16A MCB protects the 1.0mm² live conductors against faults.

- Andy.