RCD Discrimination, TT systems and Ze

[Engineer54]

Just found this paragraph in the DTI guidelines?

The PME earth cannot be taken outside the equipotential zone. This is to prevent the potential shock hazard should the supply neutral ever be lost.

Is the source of this the 17th edition somewhere - anybody?

You are not ''Exporting'' anything, you are in fact ''Extending'' the equipotential zone when there are extraneous services/metalwork etc!! Which is why you need a minimum 10mm bonding conductor in such circumstances. If no extraneous metalwork then all you require is the standard CPC....

Have you not read the ''EXPORTING PME'' Sticky thread on this forum?? Perhaps you should, it'll answer most if not all your queries on this topic....

 

[Harry Worth]

Gone to read the PME sticky thread. My Ze might be 45 Ohms - Some plonker had done what people had suggested earlier in this thread but I'm not admitting anything.

 

[Engineer54]

Gone to read the PME sticky thread. My Ze might be 45 Ohms - Some plonker had done what people had suggested earlier in this thread but I'm not admitting anything.

Did you use an extendible 5/8'' rod?? If so coupling another rod and going deeper will considerably lower your 45ohm Ra and give your rod the stability all earth rod installations need... Especially if it's now going to be used for that PV system...

 

[Harry Worth]

Hi, No, It's a 3/8 rod. I didn't think 45 Ohms was too bad as compared to the 200 Ohm figure. Grounds sodden though. What figure can I realistically aim for if that's a sensible question bearing in mind how variable I guess 'earth' is.

 

[Engineer54]

It is highly unlikely that you will have any stability with a single 3/8'' 1.2m rod!! Those things should banned full stop, total waste of money and time....

If you want to do things properly, use 2 No 5/8'' rods coupled together to give you a decent depth, which in turn should give you a decent degree of stability. It will also considerably better your final Ra value. At an outside guess based on what your getting now, i wouldn't be surprised at all, if you achieve a substantially sub 20 ohms value, and maybe better, certainly good enough for the PV system you talk about being installed at a later date.

Don't even consider comparing with the 200 ohm figure that's banded about, That's just a meaningless farcical value. Always aim for 10 ohms, you may not achieve it, as that will always depend on ground conditions and how much time and money your willing to invest.

Don't forget to install a protective flush earth pit over any earth electrode position!! This is foremost to protect the connection and as a permanent earth electrode position marker...

 

[Harry Worth]

It is highly unlikely that you will have any stability with a single 3/8'' 1.2m rod!! Those things should banned full stop, total waste of money and time....

If you want to do things properly, use 2 No 5/8'' rods coupled together to give you a decent depth, which in turn should give you a decent degree of stability. It will also considerably better your final Ra value. At an outside guess based on what your getting now, i wouldn't be surprised at all, if you achieve a substantially sub 20 ohms value, and maybe better, certainly good enough for the PV system you talk about being installed at a later date.

Don't even consider comparing with the 200 ohm figure that's banded about, That's just a meaningless farcical value. Always aim for 10 ohms, you may not achieve it, as that will always depend on ground conditions and how much time and money your willing to invest.

Don't forget to install a protective flush earth pit over any earth electrode position!! This is foremost to protect the connection and as a permanent earth electrode position marker...

As always every answer raises another question. I've found the 200 Ohm figure in the regs, OSG and GN3 but I recollect another figure I can't find anywhere which I remember being quoted in a table at college i.e. Ze TNS 0.8, TNCS 0.35 and TT 21 Ohms - I have searched for the source of this TT figure in the regs with no luck. If the other 2 systems were outside the parameters quoted then representations would I guess be made to the DNO yet with the TT system if 21 Ohm can't be met then this figure of 200 Ohms is mentioned. As mentioned, earth is absolutely sodden so that figure I've managed will be best possible case - when we have our next drought (probably in the next few weeks knowing our climate) ground will probably be an excellent insulator. So take your point absolutely.

I probably won't spend too much more time on it as the transformerless inverter being fitted is temporary and will only be there a short time (heard that one before). Also as I understand it, guidelines are being changed shortly so the earth rod won't be required. It's been a sort of learning exercise really - I've learnt a lot already. Trouble is the more I know the more I realise there's far more I don't know!

I bought the earth rod as part of a kit - told I had everything I needed. I've installed a black plastic box along with a warning label over it. Won't be using that kit in anger again!

 

[Engineer54]

As mentioned in my previous post, nothing stopping you from connecting this rod via the garage CU to the MET, That's what i would be doing if it was my house/property...

The 21 ohm figure you mention is the maximum Ra at the DNO's power station/substation, but will generally be much much lower than that 21 ohms....

Like those 3/8'' spikes, those black plastic boxes are also next to useless, they sit above the ground and get battered and destroyed over the years. My best advise to you is that on any future TT systems that you install, is made up of at least 2 X 5/8'' extensible screwed rods via a coupling. And the use of a ''flush'' earth pit!! ...At the very least you will then know, your earth electrode will have a fair degree of stability.

One thing to note when using coupled rods, because the coupling has a slightly larger circumference than the rods, the top rod will take time to reach it's full potential while the soil consolidates back around giving full contact. Some create a pool of water around the rod while it's being driven in and this does help, but only time will give full and complete contact...