Short circuit protection on high resistance on live conductors

[cliffed]

Thoughts on this scenario,obviously a short circuit fault situation

 

[Pretty Mouth]

Thoughts on this scenario,obviously a short circuit fault situation

Short circuit means a fault between live conductors, eg. L and N. Typically the fault would have a low loop impedance, and so a high fault current. The OCPD would likely operate, but the RCD wouldn't operate as the current is equal in L and N.

In the scenario you posted, it talks about a fault to earth on a TT system. In this case, the fault loop impedance would be quite high, the resulting current would not be enough to operate the OCPD. An RCD would be required to disconnect the supply.

 

[Ian1981]

Your example is for a TT earth fault where only a low current would flow resulting in an ocpd not operating.
If you measure the L-N loop like the PSCC on a TT , then you will find that to be typically in the low impedance values similar with doing ZE testing in a TN arrangement.

 

[cliffed]

Agree,but what happens when the loop impedance is high on a circuit,on a TN System,a Rcd/rcbo protects for an earth fault.
The L&N resistance is high too,so the short circuit current is low,& maybe the possibility of the rcbo/mcb not operating.

 

[pc1966]

There are few situations in my mind where relying on a RCD for earth-fault protection is reasonable in the TN cases, as I think the RCD should be looked at as "additional protection" and not the primary means of disconnection. A high current sub-main is the most obvious case where Zs may be too high to meet the disconnection times.

Of course in the TT case an RCD/RCBO is often the only reasonable means of disconnection and is permitted, but I would still be happier with a 100mA delay-RCD incomer and 30mA per-circuit RCBOs so there is no single point of failure with the significantly more complex electronics than the thermal-magnetic trip mechanism (or fuse) that would be OK in the low impedance TN cases.

When looking at a high impedance for a L-N fault you would not normally see that if you also meet the 5% volt drop limit on a general circuit - as that would imply something around 20*In fault current. Where it might happen is something like a fixed load of, say, outside lights where you have a 6A breaker but a very long cable as you only have 100W of LED lights and so 3% VD is met for 0.4A (i.e. 17 ohms or so) but not the PSSC for rapid fault clearing by the breaker.

Now that is not against the regs, and the cable would still be protected (assuming its thermal limit is below the MCB, etc) but it is still a bad situation. Again, my personal view is you should be designing for 5s clearance max, but that is not actually required as the ADS times only apply for L-E faults.

A smaller OCPD would solve it, but given most brands of RCBO only go down to 6A, and not all manufacturers offer MCBs below 6A either, your only option in that case might be to have a FCU as switch or isolator so you can use a 3A fuse (or even 2A or1A as they can be found from the likes of RS, etc).