TT system but no RCD as main switch

[cashnotakite]

hi,

need some help with this one;

I'm testing a board that is TT but doesn't have an RCD as main switch, it has a standard 100A switch disconnector (60947-3). However, every circuit in the installation is RCBO protected. Is this ok?

Just to make matters a little more awkward the supply cable is a 25mm single core PEN conductor with the outer sheath used as the neutral. The regs seem to be saying that RCD's shouldn't be used as there is no seperate path for earth currents, there is however a 16mm earth conductor which has been taken out to the earth electrode.

any help with this is much appreciated

thanks in advance

 

[johnboy6083]

is the unit metal clad? if yes then you require an RCD beopfe the CU, to provide eartrh fault protection to the tails.
If the unit is plastic,the yopu dont require an RCD before trhe CU, but you still require RCD's to provide earth fault protection to circuits.

 

[Engineer54]

hi,

need some help with this one;

I'm testing a board that is TT but doesn't have an RCD as main switch, it has a standard 100A switch disconnector (60947-3). However, every circuit in the installation is RCBO protected. Is this ok?

Just to make matters a little more awkward the supply cable is a 25mm single core PEN conductor with the outer sheath used as the neutral. The regs seem to be saying that RCD's shouldn't be used as ,there is no seperate path for earth currents there is however a 16mm earth conductor which has been taken out to the earth electrode.

any help with this is much appreciated

thanks in advance

So what do you think that earth electrode (Rod) is there for then in a TT system?? So what's the Ra of this TT system??

 

[Deleted member 26818]

The non-RCD protected part of the installation, must be double insulated.
So that's sheathed tails, and a plastic enclosure for the switch disconnector and the CU.

 

[JONOXNAM]

Yes TT with only RCBO's is acceptable, if I remember correctly that's given as an example in the OSG and obviously causes the least amount of inconvenience in the event of a trip, not so common due to the cost!

I don't think the regs state RCD's should NOT be used!

Remember the reason for using an RCD in a TT installation - the earth path will probably not be a low enough resistance to ensure disconnection times but reading 411.5 and 411.3.2.2 the regs DO NOT state the absolute need for an RCD - if you can ensure your return path is low enough and that it can be maintained at that value TN conditions can apply.

I have a TT supply to my house and am quite close to the sub station, with a big EC and an austenitic steel rod I have managed to get RA to 0.25ohms when I fitted it in the summer, but would I trust this to remain this low, NO, hence TT's should employ RCD's due to instability (corrosion, changes to soil etc) to provide a reliable way of ensuring continued disconnection in time... remember at the end of the day this is what you have to make compliant as an example ever had a high suppliers TNCS Ze e.g. 5 ohm and you call the local power company, they turn up and say yeah mate that’s fine, its then your problem to ensure disconnection time!! Zs=Ze+(R1+R2)!

The earthing conductor in TT might need only be 2.5mm - see 542.3.1 & 543 (16 is an easy way to make sure you cannot but comply with tails of 25mm).

Hope this helps, Jon.

 

[Engineer54]

I have a TT supply to my house and am quite close to the sub station, with a big EC and an austenitic steel rod I have managed to get RA to 0.25ohms when I fitted it in the summer, but would I trust this to remain this low, NO, hence TT's should employ RCD's due to instability (corrosion, changes to soil etc) to provide a reliable way of ensuring continued disconnection in time... remember at the end of the day this is what you have to make compliant as an example ever had a high suppliers TNCS Ze e.g. 5 ohm and you call the local power company, they turn up and say yeah mate that’s fine, its then your problem to ensure disconnection time!! Zs=Ze+(R1+R2)!

The earthing conductor in TT might need only be 2.5mm - see 542.3.1 & 543 (16 is an easy way to make sure you cannot but comply with tails of 25mm).

Hope this helps, Jon.

Your a very lucky chappy to have an Ra of 0.25 ohm With an Ra of that value any RCD protection would be classed the same as a TN earthing system, ie, ...''Addition Protection'' and NOT your main means of earth fault protection. So why would you NOT trust this value to remain stable?? Why did you use a steel electrode for your earth rod and how deep did you drive it??

If your Ra value is correct, then your TT system is going to be far more reliable than a RCD device, they are not the most reliable bits of kit at the best of times and far more susceptible to failure over time than a good TT system, ....of that i can assure you. The only doubt i have about your system, is the use of a steel rod, and that it maybe not of a suitable depth for stability purposes!!!

EDIT.... If you informed Any DNO that you have professionally measured a Ze value of 5 ohms on a TNC-S/PME system, they would be a lot more concerned than your making out here. They are NOT going to be saying to you ''yeah mate, that's fine!!'' .....That's more than they would dare say or do!! That sort of figure, and they would know they have problem that needs sorting sharpish!!!

 

[windtech]

In the summer the ground drys out and the ZE will go up. Where i live if you dig 1/2M down its sand what is even worse as it doesn't hold water

 

[Engineer54]

In the summer the ground drys out and the ZE will go up. Where i live if you dig 1/2M down its sand what is even worse as it doesn't hold water

Easily remedied, if your prepared to spend a bit of time and money... lol!!

 

[JONOXNAM]

Hi I used an austenitic rod, it was 1.25m in length (I had another 2 to drive on top but did not need them) - as the long term corrosion is greatly reduced vs copper hence it will be more stable, but in frosty or other dry ground conditions contact with the rod is reduced and Ze will increase. My main point was that even though my Ze is nice and low I still want the backup of RCD protection as there is no fixed permanent reliable connection, if cost was not an issue I would have a 100mA an all RCBO’s vs my split board.

Considering the above you can’t really say that value of Ra is more stable than an RCD but a god low Ra coupled with RCD makes it as safe as it can be.

I assure the DNO did exactly that and it is amazing as that could be a broken Neutral somewhere and all earths being raised to live potential after the break...

One point how can you unprofessionally measure Ze?

 

[SPARTYKUS]

Ze unprofessionally- measure it in the dark after 8 pints! That would be unprofessional!!!

 

[Engineer54]

Hi I used an austenitic rod, it was 1.25m in length (I had another 2 to drive on top but did not need them) - as the long term corrosion is greatly reduced vs copper hence it will be more stable, but in frosty or other dry ground conditions contact with the rod is reduced and Ze will increase. My main point was that even though my Ze is nice and low I still want the backup of RCD protection as there is no fixed permanent reliable connection, if cost was not an issue I would have a 100mA an all RCBO’s vs my split board.

Considering the above you can’t really say that value of Ra is more stable than an RCD but a god low Ra coupled with RCD makes it as safe as it can be.

I assure the DNO did exactly that and it is amazing as that could be a broken Neutral somewhere and all earths being raised to live potential after the break...

One point how can you unprofessionally measure Ze?

So, you've got another 2 X 1.25m rods, but you didn't need them, but your worried about the rods stability during freezing and drying periods??? So, didn't commonsense tell you that if you coupled and drove the other rods onto your existing rod, you wouldn't need to worry about stability??

Believe me, i would trust a stable Ra of 0.25 ohms over any RCD device that's available today on the market!! RCD's can and DO often fail, while stable TT systems Rarely if ever fail!!! Not at least for 3 or 4 decades anyway!!

Your now talking about a TNC-S earthing system, that relies on the earthed neutral conductor for it's earthing point, which is so far away from a TT system, it's not worth discussing here. Your TT system will not be affected by any broken neutrals somewhere. And the chance of having a broken neutral, on a PME system is so remote it's also not worth discussing here...

Now i'm wondering if you really do have an Ra of 0.25 ohms, if your asking how to measure Ra/Ze!!!!