If all circuits are rcd or rcbo protected max current 150mA why do you still have to install 16mm earth

[Rodrigo12]

THIS IS FOR ELECTRICAL NEWS ONLY

 

[Deleted account]

Already answered this one in another thread.

RCD is additional protection, Above and beyond fault protection.

All circuits should be designed with fault protection in mind.

The RCD is there as an after thought.

 

[Rodrigo12]

Already answered this one in another thread.

RCD is additional protection, Above and beyond fault protection.

All circuits should be designed with fault protection in mind.

The RCD is there as an after thought.

MCB will give overload fault protection earth leakage fault protection is provided by an RCD what other faults are there?

 

[Julie.]

As above RCD is additional, not primary.

Also if you have a small CPC and substantial fault conditions you could have two effects:
1) The CPC burns out on the first fault, all subsequent faults involving earth may have no connection to the actual earth, the RCD's may not even trip.
2) When you have an earth fault, it isn't just 30mA which flows, it is the full current - so say 1000A or more, if your earth loop impedance is high due to smaller section cables the voltage drop could be much higher , say you have R1+R2 0.2ohm, Ze 0.2ohm and a thin, weak earth tail of 0.3 ohm (common in old buildings) the voltage drop would be circ 100V across that tail - much of the current could still flow through a person.

Large low resistance cable cut the touch voltage down

 

[Wilko]

The earthing conductor from the Means of Earthing to the MET is often 16mm as it’s been selected from Table 54.7 in BS7671. From the table with 16 to 25mm tails it’s 16mm. But there are a few other things -

The size may be calculated from the Adiabatic Equation in Reg 543.1.3, which not surprisingly will often lead to a smaller size. In your case I would use the fault current assuming the RCD failed to operate so the earthing conductor would not be damaged if that happened.

Then there’s Reg 543.1.1 that sets the minimum size for separate earthing conductors which I take as 4mm normally. And if it’s buried Table 54.1 has us back at 16mm again.

Theres probably more .

 

[Lister1987]

As others have said; an RCD is not designed Bethe primary source of protection, simply because it requires mechanical operation to work and, like any electrical components, can fail - Simple science however does not, and this is where our friends the equations come in.

 

[Deleted account]

MCB will give overload fault protection earth leakage fault protection is provided by an RCD what other faults are there?

Short circuit.

 

[pc1966]

A typical "instant" RCD will trip in 10ms to 40ms and be in specification, or a S-type delay one in another 100ms or so.

If you hit the magnetic trip of a MCB it is usually below 10ms so the fault I2t is much lower.

Fuses can be faster still at high currents, but the time shown on the I/t plot is what is known as "virtual time" and it is really telling you the I2t level for a given PFC. So you might see microseconds of time but the fuse is actually burning for longer but has limited the current, leading to the same I2t.

 

[loz2754]

Why is this in the electrical news?

 

[Lister1987]

Why is this in the electrical news?

No idea, think th system is bork d, click the H LP Populate Electrical News Please link and I get a permission error so it wouldn't suprise of its messed u

 

[pc1966]

I had been wondering as I don't think the "news" board has seen any news as such, just entries that ought to be on the normal forum.

Do any of the moderators know if they can simply be moved to there?

 

[Rodrigo12]

As others have said; an RCD is not designed Bethe primary source of protection, simply because it requires mechanical operation to work and, like any electrical components, can fail - Simple science however does not, and this is where our friends the equations come in.

They fail open circuit in my experience.

 

[old man sparky]

Ok so now we have established the main earth sizing at 16mm is in relation to time current characteristics pertaining a potential fault current.
How about an example of where a fault current would exist of sufficient magnitude AND TIME to justify a 16mm main earth. We have already said the adiabatic equation does not support a 16mm. So how in practical terms do we justify 16mm as opposed to 10mm or even 6mm. DO NOT quote from the ruddy book.
Lets have some real answers for the OP. ??

 

[old man sparky]

They fail open circuit in my experience.

Not always. They can be in service for years and keep circuits energised. Only when tested do you hear, blimey that's lucky !! Wonder how long that's been like that ??

 

[Soi disant]

Already answered this one in another thread.

RCD is additional protection, Above and beyond fault protection.

All circuits should be designed with fault protection in mind.

The RCD is there as an after thought.

Tell that to the people who write the OSG.

 

[old man sparky]

Tell that to the people who write the OSG.

The people who write the OSG are all part of one big happy family. Masters of the Universe. All gathered around a big round table.
Example of a typical conversation.
EICR coding's for cables buried in wall less than 50mm depth. Although they would say Regulation 522.6.202-204. cus it sounds better.
NAPPIT official " code 2 old boy"
NICEIC "No No old chap, its a code 3"
NAPPIT "No definitely code 2"
NICEIC "I beg to differ sir !! code 3.
NAPPIT "Stuff you !! we are putting code 2 in OUR book.
NICEIC "Well stuff you too !! We are putting code 3 in ours.
And therein lies the story ?

 

[pc1966]

How about an example of where a fault current would exist of sufficient magnitude AND TIME to justify a 16mm main earth.

Open PEN fault and you have bonding to both gas and to water which are metal and complete the neutral return for your own, if not a few, properties.

Currents to many tens of amps, duration as long as it takes for the DNO to find out and act.

 

[pc1966]

Or going back to basics and a L-E fault at the CU. A quick look at OSC Table B4 or B5 shows with a 100A fuse it would fail adiabatic for 10mm at around 0.27 ohms Zs.

 

[old man sparky]

Open PEN fault and you have bonding to both gas and to water which are metal and complete the neutral return for your own, if not a few, properties.

Currents to many tens of amps, duration as long as it takes for the DNO to find out and act.

So nothing to do with fault current. We are talking about a bad design which allows LOAD current to bypass all over current devices a way thats completely out of the control of the consumers side of the installation. A bit like a 5 amp rewirable fuse getting warm due to excessive load so the answer is put a 20 amp fuse wire in. Over simplistic analogy I grant you. But hope you guys can see where I have guided this one. You have answered the question put by the op which is exactly correct. Nothing to do with fault currents all to do with load currents created by bad design DNO side and BS 7671 puts its finger in the dam. Sorry for the rant but it annoys me the way these knee jerk reactions are given credibility, and its the sparky in the field left holding the Hot potato.

 

[pc1966]

While I agree with you that TN-C-S was a dumb approach, it is still a fault current.

Just not your fault...