RCDs for fault protection in adibatic equasion

[samn420]

Hi all

Was looking in to the adibatic equasion as our company will not let us add on to a Ring circuit with 1mm cpc..

I want to give some evidence as to why it may not be an issue.

My question is;

If the circuit is protected by an rcd can I replace fault current in the adibatic equasion with 30ma as that is all the rcd will potentaly allow to flow down the cpc?

Thanks

 

[James]

Rcd will not limit fault current, only limits the time it is flowing.

 

[samn420]

Rcd will not limit fault current, only limits the time it is flowing.

I thought an rcd will trip at 30ma witin 300ms surly that is limiting the fault current correct me if I am wrong tho

 

[samn420]

So I could replace the 0.1 trip time in the equasion with the actual measured 1x trip time of the rcd ?

 

[James]

If you have a low resistance fault from phase to earth.
the current that flows will be dependent upon the loop impedance.

if this causes an imbalance through the rcd over 30mA (it should) then the rcd should open to clear the fault.
It will not limit the current that flows as it has no current limiting circuitry inside it other than a trip mechanism to turn off the supply to the load.

 

[samn420]

If you have a low resistance fault from phase to earth.
the current that flows will be dependent upon the loop impedance.

if this causes an imbalance through the rcd over 30mA (it should) then the rcd should open to clear the fault.
It will not limit the current that flows as it has no current limiting circuitry inside it other than a trip mechanism to turn off the supply to the load.

Thanks James

So I could replace the 0.1 trip time in the equasion with the actual measured 1x trip time of the rcd ?
Also I'm struggling to know where exactly to take my fault current from. The start of the circuit where fault current will be at it highest. Or just simply from my zs of thay circuit

 

[Pretty Mouth]

It's often possible to prove that a 1mm CPC will comply with the adiabatic for a ring final. What OCPD is protecting the circuit? Also manufacturer

 

[pc1966]

I thought an rcd will trip at 30ma witin 300ms surly that is limiting the fault current correct me if I am wrong tho

No it is limiting the time above 30mA. You might have 1kA flowing for 40ms or more!

 

[pc1966]

In general the RCD will only provide better (i.e. lower I2t energy) fault limiting for the region below the MCB "instant trip". So if you have your typical 32A B-curve MCB then it will go in under 20ms for any current above 5*32 = 160A and the speed of disconnection (below a cycle of the supply) provides the energy-limiting aspect.

Below that magnitude of fault current then the MCB can take seconds to trip on the thermal curve, but the RCD for earth-faults will go in tens of milliseconds instead. So while it might stop the CPC slowly roasting at 30-odd amps under very odd fault conditions it is rarely the aspect that will decide on CPC minimum size.

For UK plugs the 13A fuse also serves to limit long-term overload on such faults (e.g. very long extension lead shorted out) but these days the RCD ought to disconnect long before that.

 

[samn420]

It's often possible to prove that a 1mm CPC will comply with the adiabatic for a ring final. What OCPD is protecting the circuit? Also manufacturer

32a rcbo starbraker

 

[oscar21]

I has this discussion the other week, someone was saying an RCD will save someones life because it limits the shock to 30mA. I said you still get the maximum current the body can pass due to its resistance which may be way more than 30mA, the saving grace is it wont be for very long.

I think it was something to do with the difference in time between an MCB tripping and an RCD tripping on a shower circuit, I said there was hardly any difference between the two if an earth fault occurred on the circuit.

 

[Pretty Mouth]

32a rcbo starbraker

For starbreaker MCBs I only have limited I²t data unfortunately:

For a B32, fault current of 6kA, the I²t is 31760 A²s. Gives you a CPC of 1.5mm² once you've run it through the adiabatic.

I expect the Ipf at the CU is lower than 6kA, which would probably allow a smaller CPC, if we had the I²t data to prove it. But we don't. That is frustrating.

 

[pc1966]

I expect the Ipf at the CU is lower than 6kA, which would probably allow a smaller CPC, if we had the I²t data to prove it. But we don't. That is frustrating.

Most likely it would be fine, as it is fairly certain the I2t drops with PFC (at least until thermal region) and 6kA is quite high for a domestic CU.

Also for a RFC you ought to have two CPC in parallel, though near the ends (close to CU) it will mostly be current in one leg, and higher PFC as well, so not a good basis for such an argument!