Consumer unit with RCBO, grounding necessary?

[Gulfsailor]

Hi,

A relative is moving into a small house, part of many connected houses. There’s a 5(15)A meter at each house (not from the PEA, but from the landlord), and I have no idea what it looks like before that point. There is no earthing anywhere. Inside the house is only a simple single MCB with everything connected to it. 
so I want to add some safety and install a consumer unit with RCBO. The lightest one I could find is 32A with 4 CB’s from 10 to 20A.  

I do have some questions;

- do I keep the original MCB and install the CU after it, or is there really no need for that?

- the wiring schematic wants the E and N connected. There is no grounded wiring or a ground rod (at least after the meter). There will be a small aircon installed later though. Would adding a ground rod only increase safety in case of neutral failure? But in that case the RCBO would catch it when someone links L to E right? So the only benefit of the ground rod (and wiring) is that the RCBO would trip at leakage even when no one is touching the live?

- is there anything to take into account if I do decide to install a ground rod? Or just connect it straight to the earth bar in the CU?

 

 

 

[sometimewoodworker]

Ground is always needed, an RCBO is not a replacement for ground.

 

There are faults that will not trip an RCBO that can kill.

 

So simple answer add grounding and an RCBO / RCCD but get a real Electriction if you can, as it is perfectly possible to make it more dangerous if you get things badly wrong 

Edited June 24, 2022 by sometimewoodworker

[Crossy]

Yup ^^^. Now whether you link N-E is another animal.

 

Check the supply poles. If the heutral (the tope wire if the 2 or 4 on small insulators) has a ground connection every 3rd or 4th pole then the area is wired as MEN (Multiple Earthed Neutral) and a N-E connection will add another layer of safety.

 

If you don't see that earthed neutral or you are not sure then assume that the supply is TT and run the neutral straight into the RCBO.

 

Any Class-1 appliance (water heater, A/C, most white goods) will want an earth to prevent that annoying "tingle" so many of us are familiar with here.

 

[longball53098]

I think he  wrote he would be adding a aircon later. He also said the landlords meter is only a 5/15 meter so the supply might be stressed a bit ,,,,,,,yes?

[sometimewoodworker]

1 hour ago, longball53098 said:

I think he  wrote he would be adding a aircon later. He also said the landlords meter is only a 5/15 meter so the supply might be stressed a bit ,,,,,,,yes?

Stressed? No. A 5/15 meter can quite happily supply 25 amps all day long, however it is out of the guaranteed accuracy range & if inaccurate it will be reading more power consumption than is actually being used.

[Crossy]

Also, a 12,000 BTU A/C will only pull about 5.5A when running at full chat.

 

Of course if he's going 30,000 BTU then ...

[Gulfsailor]

Quick update. Managed to install the CU. Couldn’t cut the power without putting the whole row of houses in the dark, but just capped the live wire off and was extra careful, even wearing rubber sandals while standing on a wooden chair ????. There was no sign of grounding anywhere, so I connected the N directly to the RCBO. It took a while to figure out why one group kept tripping the RCBO, but some genius had wired the porch light with the switched live wire coming from inside and the N going directly to the meter, bypassing the CU. All good now.  
ps, the aircon is a 12k BTU inverter in a tiny bedroom, so should be fine on a dedicated 10A group. 

[Crossy]

@Gulfsailor glad you got it sorted without mishap.

 

I would however caution anyone contemplating working "live", get a pro or a local in.

 

It's not actually the risk of getting a shock which is the greatest hazard (it is of course a hazard) but the danger of "arc flash" should your incoming supply live contact the incoming neutral. There's no over-current protection whatever other than the drop fuses at the village transformer!

 

An arc-flash event can prove fatal or life-changing without the proper protective gear.

 

https://en.wikipedia.org/wiki/Arc_flash

 

[Gulfsailor]

Okay, so the aircon guys installed the aircon and we’re so kind to put a ground rod in. Initially they just wanted to connect it directly to the aircon but they agreed to go through the CU, so other future grounded equipment can use it as well. 
my question now; do I keep the ground separate from the neutral in the CU, or run the incoming N via the ground bar to the RCBO, as it’s indicated in the wiring diagram? Wouldn’t connecting it mean that if a live wire touches a metal casing the RCBO doesn’t trip, since it’ll still flow to the N before going out through the RCBO? Or does the ground have less resistance than the N, so it prefers that route?

[Crossy]

As my earlier post :-

Check the supply poles. If the heutral (the tope wire if the 2 or 4 on small insulators) has a ground connection every 3rd or 4th pole then the area is wired as MEN (Multiple Earthed Neutral) and a N-E connection will add another layer of safety.

 

If you get a L-E fault when N-E are linked the breaker will still go out as one side of the supply still goes through.

 

A L-E fault on the incoming L is bad news whichever way you cut it ???? 

 

 

[Gulfsailor]

4 minutes ago, Crossy said:

As my earlier post :-

Check the supply poles. If the heutral (the tope wire if the 2 or 4 on small insulators) has a ground connection every 3rd or 4th pole then the area is wired as MEN (Multiple Earthed Neutral) and a N-E connection will add another layer of safety.

 

If you get a L-E fault when N-E are linked the breaker will still go out as one side of the supply still goes through.

 

A L-E fault on the incoming L is bad news whichever way you cut it ???? 

 

 

There’s no MEN or any ground anywhere to be seen. Apart from the new one now which goes to my CU ground bar. 
I’m still not understanding the need to connect the neutral and ground bar in this case. If there’s a L-E short after the CU, wouldn’t the electricity then just flow back through the E from the appliance to the CU, and from there via the N through the RCBO? Meaning it won’t trip? Or would part flow from the CU to the ground bar, so the RCBO would trip?
 

 

[Crossy]

Have you checked if your incoming neutral is grounded at the poles (every 3rd pole or so) for Multiple Earthed Neutral (MEN)?

 

The idea of TNC-S (with or without MEN) is that it provides a metallic (low resistance) path back to the transformer star point that will trip the over-current protection without anything going through you. Adding MEN means that the hazard from a failed TNC-S neutral is reduced.

 

There is nothing wrong with going TT like you have now but you are relying on the RCBO earth leakage protection rather than the infinitely reliable (read simple) over current protection.

 

[Gulfsailor]

6 minutes ago, Crossy said:

Have you checked if your incoming neutral is grounded at the poles (every 3rd pole or so) for Multiple Earthed Neutral (MEN)?

 

The idea of TNC-S (with or without MEN) is that it provides a metallic (low resistance) path back to the transformer star point that will trip the over-current protection without anything going through you. Adding MEN means that the hazard from a failed TNC-S neutral is reduced.

 

There is nothing wrong with going TT like you have now but you are relying on the RCBO earth leakage protection rather than the infinitely reliable (read simple) over current protection.

 

There is no MEN. 
so, do I connect the incoming N through the ground bar in the CU, or not?

[Crossy]

1 minute ago, Gulfsailor said:

There is no MEN. 
so, do I connect the incoming N through the ground bar in the CU, or not?

 

If there is no neutral grounding at the poles don't route the N via the ground bar.

 

[Gulfsailor]

Just now, Crossy said:

 

If there is no neutral grounding at the poles don't route the N via the ground bar.

 

Okay, thank you!

[Crossy]

Just now, Gulfsailor said:

Okay, thank you!

 

Welcome, I did say all that in my very first post on this thread ????