Just wondering

[Julie.]

Agree, it should disconnect sooner rather than later. But it is a struggle to find a circuit where SC loop impedance would the limiting factor, at least for the type of work I do. Voltage drop almost always gets there first. Very lightly loaded, very long lighting circuits perhaps.

The OSG paragraph is in my previous post #24, but doesn't link to a regulation, and I can't find anything in the regs that directly back it up.

Well I actually dug my osg out last night, dusted it off and cleaned the cobwebs away, and yes it's in there!

That note and looking at the table suggests the loop impedance is the most common limiting factor without a rcd, and my experience would be the same, although the only real occasion in my experience is long remote lighting with small loads.

I don't know of a regulation which determines a required trip time, but obviously there's a subsection for fault protection which doesn't seperate a need between line-line, line-neutral, or live-cpc/earth, basically demanding protection for all types of fault current, whist live-cpc/earth faults do have a specific set of disconnection times

 

[Pretty Mouth]

Well I actually dug my osg out last night, dusted it off and cleaned the cobwebs away, and yes it's in there!

That note and looking at the table suggests the loop impedance is the most common limiting factor without a rcd, and my experience would be the same, although the only real occasion in my experience is long remote lighting with small loads.

I don't know of a regulation which determines a required trip time, but obviously there's a subsection for fault protection which doesn't seperate a need between line-line, line-neutral, or live-cpc/earth, basically demanding protection for all types of fault current, whist live-cpc/earth faults do have a specific set of disconnection times

Just thinking out loud:

434.5.2
A fault occurring at any point in a circuit shall be interrupted within a time such that the fault current does not cause the permitted limiting temperature of any conductor or cable to be exceeded.

it then goes on to give a rearrangement of the adiabatic to calculate that time. The adiabatic is only accurate for faults lasting up to 5 seconds. Perhaps this is why the OSG wants us to limit the duration of any fault to 5s?

 

[D Skelton]

Zs is measured to determine whether or not fault protection is achieved. The requirements for fault protection do not concern themselves in any way, shape or form with short circuit protection.

Section 434 is where you will find the requirements for disconnection under short circuit conditions.

To answer the OP's original question, 1667 ohms is the correct value as the sole means of fault protection is the 30mA RCD. If stable TN values have been achieved then the correct value to input would be 7667 ohms.