Stick to TT or go PME?

[FatAlan]

Ok, so you have an old non RCD protected domestic installation on a TT earthing arrangement.
Would it be preferable to upgrade to PME if available by the DNO or stick to TT and protect with up front 100mA RCD if the Ze was ok via the rod?

 

[FatAlan]

Also with rumours just before the 18th coming out of a move back to requiring earth rods. Was that a sniff of the big boys in the private sector wanting to avoid responsibility for providing an adequate route to earth?

 

[l4urence]

Also with rumours just before the 18th coming out of a move back to requiring earth rods. Was that a sniff of the big boys in the private sector wanting to avoid responsibility for providing an adequate route to earth?

I think you have a point. The issue with PME is the open circuit PEN conductor - hence the requirement for a rod for PV, outbuilding, shed. So if someone gets fried it’s our responsibility not the DNO, why? Smart meters can shed the load by switching off the supply (volatile renewables?) like I said, it’s all changing. I like TT, you know where you are. My only gripe is the need for a type 1 SPD which are not cheap.

 

[Markyd]

How very bizarre. Not seen that in my neck of the woods, some changing of PME back to TT for EV charging. The world is changing.

I think most EV points have to have their own TT in place

 

[l4urence]

I think most EV points have to have their own TT in place

Only if the supply is PME

 

[Wilko]

Hi - domestically where gardeners may cause Ze fading with a spade, I would convert to PME and RCBO.

 

[davesparks]

When I can, I change TT CU’s to RCBO’s. I don’t really like one central RCD that trips the entire installation. I don’t really get how the overhead TT line could be changed to PME, certainly not in my location.

What's not to get? A lot of overhead lines installed since the 70's will be PME supplies. I don't know if this is universal but in the SSE area every pole that has an earth electrode and N-E link has a label on it with the letters PME in red on a white background. So the availability of PME is easy to check.

One very well respected former member of the forum insisted that only overhead supplies can be true PME supplies.

 

[davesparks]

Ok, so you have an old non RCD protected domestic installation on a TT earthing arrangement.
Would it be preferable to upgrade to PME if available by the DNO or stick to TT and protect with up front 100mA RCD if the Ze was ok via the rod?

In this area a lot of overhead PME supplies have a label on the poles where the N-E link have been made, usually red letters on a white background about halfway up the pole. It's an easy way to check the availability of PME

 

[davesparks]

SSE have changed a couple for me both on overhead supplies. All they appear to do is check the available Ze. That’s why I asked the original question really as my DNO SSE appear to be quite happy to swap to PME.

All they do on site is check the Ze, someone in the office will already have checked if the overhead lines in the area have been fitted with the required N-E links etc for PME.

 

[Markyd]

Had mine changed to PME

 

[l4urence]

What's not to get? A lot of overhead lines installed since the 70's will be PME supplies. I don't know if this is universal but in the SSE area every pole that has an earth electrode and N-E link has a label on it with the letters PME in red on a white background. So the availability of PME is easy to check.

One very well respected former member of the forum insisted that only overhead supplies can be true PME supplies.

Fantastic answer! Thanks @davesparks - I’m going to be looking at this moving forward. What I didn’t get was how WE can change the installation to PME, surely that needs to go through the DNO.

 

[telectrix]

SP will convert to pme if available. last one i had them to they came same day. no charge. they connected N-E in the head and fitted a 16mm tail from there into a henley.

 

[davesparks]

Fantastic answer! Thanks @davesparks - I’m going to be looking at this moving forward. What I didn’t get was how WE can change the installation to PME, surely that needs to go through the DNO.

Yes the DNO need to make the final connection to the head, what we do is prepare the installation to be connected to PME by ensuring the earthing conductor and any bonding are the required size and anything else that needs to be done.

 

[Nigel]

I had this argument with the NWEB years ago (long before most of you were born). A TT supply the lecky board wanted to make PME, no bloody way! My reasoning, they were wanting to use my “earth nest” for free. It was far better than their puny rod.

UKPN tried the same trick years later on an 11kV system. The earth nest was 100’s of cast iron piles driven eight foot in to the ground. Somewhere in the order of ½ a mile feet squared in area.

Hi Tony.

 

[Nigel]

I think you have a point. The issue with PME is the open circuit PEN conductor - hence the requirement for a rod for PV, outbuilding, shed. So if someone gets fried it’s our responsibility not the DNO, why? Smart meters can shed the load by switching off the supply (volatile renewables?) like I said, it’s all changing. I like TT, you know where you are. My only gripe is the need for a type 1 SPD which are not cheap.

A Type 1 with a minimum rating of 12.5kA too.