AMD 3 DB on a TT system

[chris day]

Hello Guys

being a Londoner i have not had a great deal of experience with TT systems in a domestic setting.

The existing installation is in the basement, the existing earth rod is by the DB and sticking out of the ground, not inspection pit is that quite bad?
I was thinking of changing the rod and installing a pit, how long should the rod normally be to make sure i get a good reading.. on average?

all the final circuits will be 30ma RCD protected however with the DB being metal would i need to install a time delayed RCD before the DB?

 

[Ian1981]

If you think a non fire containing 100a main switch alongside the fire containing CU that is serves makes any sense whatsoever then crack on.
But IMO you are wrong.

On it’s own no I do not.
Thanks I will ‘crack on’

 

[Wilko]

... Existing Rod gives a healthy reading 0.30 ...

Hi Chris - my Internet diagnosis is you've a TNCS to which a rod has been added, or an old TT that has been PME'd and rod left in place. I say this because its unlikely you've a got a 0.3 Ohm Ra from a rod only, but very likely that's a number from TNCS / PME. If you'd like to confirm which system there are other tests you can do.

Ha - just reading thread - I see this has already been covered by others - apologies

 

[Deleted member 26818]

My thinking is this:
This requirement not only specifies the material the equipment is made from, it also specifies the British Standard it must comply with.
As such if the stand alone RCD and enclosure does not comply with the British Standard, then the material it is constructed from is immaterial.

When determining whether something is similar, what criteria should be used?
The description of a DB/CU in Section 2 could cover all manner of things, including for instance an FCU.
Should all FCUs now be made of metal or covered by a metal box?
An FCU is covered by a different British Standard to the one which covers DB/CUs. So perhaps the criteria used should be the British Standard.
If an item complies with the Standard which covers DB/CUs then it is similar.
If an item does not comply then it is not similar.

 

[Deleted member 9648]

That's a cop out though in reality. The main reason for the requirement for non combustible DB/CU's was poor main switch terminals and poor connection of incoming tails. To install the same main switch to the same tails outside the DB/CU in a combustible enclosure totally defeats the point of a non combustible DB/CU.
Professional electricians are required to use their judgement as well as comply with regulations, and to consider the reason for requirements when carrying out an installation. Putting a main switch in a plastic enclosure next to a non combustible DB/CU is not a professional way of working with respect to the requirements for CU's in dwellings.

 

[Ian1981]

Was it just because of the tails or was it loose connections generally including poorly connected/tightened busbars?

An isolator fitted by the DNO is in a plastic enclosure yet we are to comment on the distribution equipment when doing an eicr so to say that the DNO follow different regulations is meaningless in my view

 

[Des 56]

I don't buy the argument that a stand alone Rcd has to have a metal enclosure but do agree with the extra sentiment expressed

Professional electricians are required to use their judgement as well as comply with regulations,

Using that professional judgement I would under no circumstances fit a metal enclosure to house a isolator or Rcd on a TT system that can fault and be devoid of adequate protection

 

[Deleted member 9648]

I'm out on this one, the logic of fitting a high current main switch in a plastic enclosure in front of a non combustible DB utterly defeats me. Laughable. In fact I believe that if such an arrangement was installed and was the cause of a fire the liability of the installer would be very closely scrutinised in court.

 

[Deleted member 26818]

I'm out on this one, the logic of fitting a high current main switch in a plastic enclosure in front of a non combustible DB utterly defeats me. Laughable. In fact I believe that if such an arrangement was installed and was the cause of a fire the liability of the installer would be very closely scrutinised in court.

Thing is, if you want to apply this Regulation to the up front RCD enclosure, then the enclosure will not only have to be made from metal, the combination of RCD and enclosure will have to be manufactured in accordance with BS EN 61439-3.

 

[rolyberkin]

Anyone seen that film groundhog day!?

 

[Sparkingmad]

Where is the meter & cut out in relation to the consumer unit? Is the c/u housed under wooden stairs or in a combustible material housing?
I agree that Amd 3 came about as a result of terminals coming loose over a period of time and not necessarily due to shoddy workmanship but poor terminals used in consumer units. That is why every board and mcb/rcbo have max torque settings as overtightening was just as bad as undertightening. Also was it not the death of the brave firefighters in Southampton which kick started the whole AMD3 boards and the need for metal containment systems for all wiring in or along escape routes where cabling could fall down in the event of a fire? Which is why you code a C/U if mounted on constubistle material ie wooden board under wooden stairs or in a wooden cabinet?
I have seen an rcd go up in flames when it failed fortunately that was in an ip65 enclosure on the outside of a building. So having a nice metal non combustible board will contain the flames but that enclosure will get so damn hot if mounted on a wooden board my guess would be it could still scorch and catch fire.
Some other electricians in another post mentioned that if you have a time delayed rcd upfront of RCBO’s and you have a N-E fault on a circuit the RCD will trip and not RCBO, I have never experienced this but others were adamant this was the case.
So if that indeed is true you might just as well fit a dual rcd board and replace m/s for your time delayed 100 m/A as long as your tails go through a proper gland with rubber or foam insert to ensure they don’t touch any metal work.

 

[rolyberkin]

Or fit an SBS double pole rcbo board, fit a nice tails grommet and all sorted. What I find is laughable with the whole AMD 3 board thing is that a DNO will come along and slap in a double isolator switch in just below your nice new AMD 3 board in a plastic housing!

 

[1Justin]

Since you are getting 0.3 you are presumably measuring the neighbors decent TN earth via the bonding (repeat with EC isolated. Low reading though suggests DNO might well be able to provide PME. Why not ask them if it's available and make half your problems go away?