rcd

[jimes]

can anyone explain how an upstream supply fault can make an rcd trip?....shakey maybe

 

[spark-doctor]

An RCD protects by constantly monitoring the current flowing in the live and neutral wires supplying a circuit or an individual item of equipment. Normaly, the current flowing in the two wires is equal. When an earth leakage occurs due to a fault in the circuit or an accident with the equipment, an imbalance occurs and this is detected by the RCD, which has a sensing coil inside which automatically cuts off the power before injury or damage can result.​

 

[jimes]

that explains the load side of the rcd but supply problems can cause trips, high ze on a tt i think, just wanted to know if this was poss

 

[ezz]

I think a high Ze on a TT will do this but only if you do not have an RCD as your main isolator. If it is a TT you are using, a higher Ze is permitted but not any higher than 100 ohms (a suggested resistance by Code of Practice for Earthing) and the isolator should have a 100mA rating to compensate for this.

 

[dixon9]

From Understanding RCD´s by John Ware:

Operation of an RCD can occur due to a downstream fault such as a crossed neutral on a split-load board, high protective current or an upstream effect such as mains-borne disturbances.

Faults existing upstream of the RCD

13. A loose connection upstream of the RCD such as at the main switch or at the busbar connections can cause the device to operate.

14. Mains-borne disturbances such as spikes, voltage surges and dips, a lightning strike and the operation of distribution network switchgear and protective devices combined with capacitance to earth within the installation can cause unwanted RCD operation. A filter may be of assistance.

15. Site machinery or plant and installed services can cause mainsborne interference. Motors such as lift motors, control gear for discharge lighting and transformer inrush currents can cause unwanted RCD operation. Although significant transients can arise within an installation they would normally only occur under fault conditions. They might, however, travel to other installations where they could cause unwanted tripping of an RCD.

16. Overhead lines. Unwanted tripping may occur more frequently in an installation supplied by overhead lines compared to one supplied by an underground concentric cable. An underground concentric cable is, by its very nature, a good attenuator of transient overvoltages. Spurious tripping may be avoided by installing a filter upstream of the RCD at the origin of the installation.


http://---------------/Publish/WireRegs/WiringMatters/Documents/Issue19/2006_19_summer_wiring_matters_understanding_rcds.pdf

 

[wayne]

good informative linkfrom dixon 9 thanks

 

[LukeScotty]

i might sound dumb, but could someone explain what an "upstream supply fault" is

 

[jimes]

yeah thanks, that was what i was looking for, i wonder if large barriers on train crossing near house could cause this?...........luke upstream being before installation, main fuse back towards transformer....

 

[wayne]

how large are the barriers ? typically they only have a small motor

 

[ONIT]

which way is upstream and down stream in the supply chain

cheers

 

[jimes]

as above onit. its the outlaws house couple hundred yards from a main line crossing which has overheads aswell. Ive not looked at it yet just phone conversations tried the usual leaving lighting mcb's off, you know what its like over the phone talking to someone with no knowledge of electrics. Next door seems to be tripping alot though aswell. They left lighting mcbs off in the night to find the intruder batt went flat!no markings on mcb's!!

 

[Grae79]

being the end users everything's up stream right up to the power station, therefore everything else is down stream from there...ye olde water analogy

 

[dixon9]

which way is upstream and down stream in the supply chain

cheers

Just to back up grey´s post:

Imagine an RCD was installed to the side of a current CU/DB (due to space constraints in the CU or the need to protect a new installed circuit to the 17th ect etc...) THEN UPSTREAM would be the cable taken from the CU, the CU to the meter etc etc...

Downstream would be the cable from the RCD for the circuit, the socket outlet (for example).

Think of it as a river - the source from the hill, downstream to the river, river to streams etc etc...

 

[LukeScotty]

cheers dix's, nice way of explaining it