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Posted By Topic: RCD protection on submains

codaxx
Jun 30 2018 20:54

Just a question,

I know an RCD for submains is not required, but considering a digger drivers life, even though i have additional protection (mag slab), 50mm of bedding and then half the trench depth, lined the trench with danger tape.

still thinking of the possibility of a digger driver was to go through all 3 and hit a cable, an RCD would operate quicker than the HRC as it detects the imbalance.

Only thinking about it due to there being three phase underneath the ground.

so question, would anyone recommend protecting a three phase submains with RCD + HRC installed instead of just HRC? or am i just going over the top with protection?
I saw a 4pole RCD so thought that it would be an interesting add to a submain for additional protection.

cheers
   

ShaneR
Jul 01 2018 08:30

I have done a job as you have suggested with a submain protection via 3 phase RCD.

I\'ve had no problems so far but on reflection I think at some stage I will have problems because the sum of leakage will increase over time and start causing false tripping?

It was for a cowshed effluent pond submain Control Panel, 3 phase motors and PLC etc

I didn\'t trust the farmer not to dig up the cable with a digger or something.
   

mowgli
Jul 01 2018 09:04

Given that this extra protection is not mandated, the 30mA limit doesn\'t apply.

Since your specific aim is to clear a fault following damage by excavator you could have used a 100mA RCD.

Personally I wouldn\'t have installed the RCD.
   

codaxx
Jul 01 2018 10:07

Ah right, discrimation between RCDs is another factor.

I have not yet installed an RCD, just thought i would gather ideas of what others thought.

But in theory, if a digger was to hit the mains cable, the bucket of digger is metal, cable is surrounded by earth, the fault should run through the bucket and into earth and through earth electrode back in SWBD so it should blow the HRC regardless.
   

DougP
Jul 01 2018 10:40

Codaxx. A couple of points.

RCDs can\'t normally provide discrimination.

The bucket digger touching an active and the ground, would probably not have enough current flow to blow any circuit protection. Depending on the soil conditions of course.

I think you\'re looking for a solution where there\'s a very low likelihood of a problem occurring. The chances of them damaging the cable without touching the neutral, or shorting two phases, must be very low.

The real solution is to install a screened cable.
   

mowgli
Jul 01 2018 10:43

Another topic offered anecdotal observations of ground resistance averaging 16-35 ohms. How big are the HRCs?
   

Sarmajor
Jul 01 2018 16:45

I have removed RCD protection from submains due to the sum of the leakage exceeding the tripping current of a 30mA RCD.

You could use a 100mA RCD for this type of job as it is not a mandated requirement.

Realistically the chances of a digger driver suffering anything other than accute embarrassment if he digs up a buried live cable is very low.

Usually the digger bucket chomps through enough of the cable to cause a phase to phase or phase to neutral fault via the metallic bucket and this trips the protection. Sometimes there is not even a flash or bang that the operator notices.
The first indication they will get is seeing bits of cable hanging of the teeth of the bucket.

Extremely unlikely that you will have to rely on the resistance of the dirt earth path to operate protection.
   

codaxx
Jul 01 2018 17:29

as DougP suggested, a neutral screen would provide a lot better protection as it would give a higher chance of a neutral to phase fault. Only issue, i have had to run the cable through the groud and then up through a building, seperate conductors provide a greater amount of flex so decided against neutral screen.

Although the cables are stacked on each other underground, was just thinking worst case scenario if digger driver was to go through just one cable and if ground resistance was high, so HRC did not trip, then theres a live cable laying in the ground or creating a live digger


   

Sarmajor
Jul 01 2018 17:58

The digger will,only be alive while the bucket is in contact and in the ground. The digger if metal tracked will be very well connected to earth.

Very rare to have a digger only get one cable of a group of cables.


   

codaxx
Jul 02 2018 07:43

Thanks for the input and clearing that up.

Lets hope that digger drivers see the wanrnings before hand.