UK RCD Protecting Distribution Cables on a TT on Industrial Estate

[Ben Murray]

Hi All,

I'm looking to pick the brains of experienced engineers who have dealt with TT more than I have please.

We have an installation on a site which may or may not be unique. There are four 12m x 12m units inline with each other, all connected as one building, with the plan to build a larger, separate unit in the near future, which will be used as a self store building, so 5 units eventually.

Unit one (Landlord) is the main hub for the site, so the TP&N DNO supply enters unit one, then the tails go into a Crabtree Loadstar DB. There is an earth rod for good measure, but the steels are the main path to earth with a reliable Ze in the summer during dry months of less than 2 ohms.

The Crabtree DB is essentially being used as a feeder pillar, this supplies the following:

• Single phase to a DB within unit one which supplies all of the Landlords office equipment
• Three phase supply to the remaining units
• Eventually a three phase supply to the new building

The Crabtree DB is (as expected) under some stairs with no room to add other items on the wall, we could upgrade the DB if desired, but we cannot get any more enclosures anywhere, so this is not an option.

None of these distribution circuit supplies have RCD protection, the sub main DB's all test out fine, all 30mA RCBO's so we're happy with this. Because we're due to install a new distribution cable to the new building, my understanding is this would need RCD protection, as should the previously installed supplies (roughly 2017 when the estate was built).

Now to my question -

Firstly, am I correct with my thinking? (I know I am, but I really can't find much information and it certainly isn't something taught much these days)

Secondly, Would you:

• Install a time delayed 300mA main isolator in the Crabtree DB? It is AC type - I think that is still fine for an incomer.
• Install a time delayed 300mA main isolator in the units Pre-DB? But this doesn't protect the distribution cable.
• Install a time delayed RCCB within the Crabtree DB (which doesn't exist, so it would be a DB upgrade to a Hager or something similar), this would protect the individual cables and supplies.
• Do something completely different? Please educate me

We've been asked by the landlord to do out best to make the installation to 7671 where possible, so I'm trying to offer options, however, I have to be concious of the space (not) available.

Sorry for the long read and I appreciate your time, thank you.

 

[davesparks]

Unit one (Landlord) is the main hub for the site, so the TP&N DNO supply enters unit one, then the tails go into a Crabtree Loadstar DB. There is an earth rod for good measure, but the steels are the main path to earth with a reliable Ze in the summer during dry months of less than 2 ohms.

What size DNO supply? Is it definitely a TT and not a TN supply with an end of main electrode connected to the cutout?

How much less than 2 ohms is the Ze?

None of these distribution circuit supplies have RCD protection, the sub main DB's all test out fine, all 30mA RCBO's so we're happy with this. Because we're due to install a new distribution cable to the new building, my understanding is this would need RCD protection, as should the previously installed supplies (roughly 2017 when the estate was built).

What is recorded on the installation certificate from when it was built?

Firstly, am I correct with my thinking? (I know I am, but I really can't find much information and it certainly isn't something taught much these days)

Not necessarily correct no, fault protection by RCD is only required if Ra is not low enough to ensure ADS is complied with in the usual manner. It sounds like the Ra may well be low enough in this case.

 

[pc1966]

While it is not an explicit requiem to use a RCD in the TT cases, in most circumstances the guaranteed earth impedance via the rod or steel foundations is not low enough to meet the 1s disconnection needed on any sub-main (distribution) circuits when TT earth is in use. So in practice you will most likely need an up-front delay RCD.

You might measure the DB Zs and find it is quite low, but if you see it as very low such as 0.2 ohm or less then probably you are relying on service pipe bonding which is not acceptable as in the future such pipes can be replaced with plastic and you won't be notified.

 

[westward10]

What do you mean "earth rod for good measure"?
What do you mean "by the steels"?
If it is TT I am surprised RCD protection is not in place.

 

[James]

Unit one (Landlord) is the main hub for the site, so the TP&N DNO supply enters unit one, then the tails go into a Crabtree Loadstar DB. There is an earth rod for good measure, but the steels are the main path to earth with a reliable Ze in the summer during dry months of less than 2 ohms.

Ze (External Earth loop Impedance) should be measured with main bonding disconnected to prevent parallel paths, so what is the Ze at origin?

If you have measured Earth loop impedance with all the bonding to services still connected then it is still a useful piece of information but it is not the correct test to be doing to decide what protection is required.

I suspect that a combination of 300mA time delay rcd, 100mA time delay rcd and 30mA rcd devices may be the way forward but there is far to little info to make an informed decision.

 

[MDJ]

Hi All,

I'm looking to pick the brains of experienced engineers who have dealt with TT more than I have please.

We have an installation on a site which may or may not be unique. There are four 12m x 12m units inline with each other, all connected as one building, with the plan to build a larger, separate unit in the near future, which will be used as a self store building, so 5 units eventually.

Unit one (Landlord) is the main hub for the site, so the TP&N DNO supply enters unit one, then the tails go into a Crabtree Loadstar DB. There is an earth rod for good measure, but the steels are the main path to earth with a reliable Ze in the summer during dry months of less than 2 ohms.

The Crabtree DB is essentially being used as a feeder pillar, this supplies the following:

• Single phase to a DB within unit one which supplies all of the Landlords office equipment
• Three phase supply to the remaining units
• Eventually a three phase supply to the new building

The Crabtree DB is (as expected) under some stairs with no room to add other items on the wall, we could upgrade the DB if desired, but we cannot get any more enclosures anywhere, so this is not an option.

None of these distribution circuit supplies have RCD protection, the sub main DB's all test out fine, all 30mA RCBO's so we're happy with this. Because we're due to install a new distribution cable to the new building, my understanding is this would need RCD protection, as should the previously installed supplies (roughly 2017 when the estate was built).

Now to my question -

Firstly, am I correct with my thinking? (I know I am, but I really can't find much information and it certainly isn't something taught much these days)

Secondly, Would you:

• Install a time delayed 300mA main isolator in the Crabtree DB? It is AC type - I think that is still fine for an incomer.
• Install a time delayed 300mA main isolator in the units Pre-DB? But this doesn't protect the distribution cable.
• Install a time delayed RCCB within the Crabtree DB (which doesn't exist, so it would be a DB upgrade to a Hager or something similar), this would protect the individual cables and supplies.
• Do something completely different? Please educate me

We've been asked by the landlord to do out best to make the installation to 7671 where possible, so I'm trying to offer options, however, I have to be concious of the space (not) available.

Sorry for the long read and I appreciate your time, thank you.

much more information required.

Is the supply to the first building a DNO supply or a sub main cable from another building ?
if it is a sub main what is the earthing arrangement at the meter and cutout?
what do you mean by steels?
what is the actual reading (ZE) at the first DB?
is there one rod at the first building alone, or do all 4 buildings have a rod?
Rod aside, how sure are you this is a TT system?