TT Submains RCD question

[vde-electrician]

I have a couple of questions regarding a TT installation:

1) can overhead twin and earth submain supplying a house (on catenary wire) be protected by a 100mA time delay
rcd then 2x 30mA split load in house. (as meter position /swtich fuse) is in a seperate shed/building. or does it depend on what earth fault loop impedance i get form the earth rod (next to meter)? If so, how do i calculate?

2) in a farm building, can a submain be covered by 100 mA time delay if the twin and earth is enclosed in pvc conduit up the wall then clipped direct up in the roof then dropped in conduit (not likely to get damaged in roof as its very high). and then use 30mA for the final circuits. It is assumed no livestock is being kept in this building, but would that make a difference? (this is TT installation again)

thank you
any advice is great

 

[JD6400]

More to the point , would you say twin and earth is the best choice for either environment !?

 

[D Skelton]

This should be in the DIY section surely? OP if you contact the mods they can move it there for you.

 

[Deleted member 9648]

Sounds a bit like a homework question.

 

[vde-electrician]

1)no i would not say twin and earth is the best for an overhead - but this is existing and may have been installed correctly at the time of its regs. I have worked alot on National Trust properties and you find these alot. Perhaps you just work new installations mostly? Done many condition reports containing overhead cables - Advised by NIC inspectors - they are acceptable , providing they are supported correctly and of a suitable height etc . i just was unsure on rcd protection of them? Obviously swa would be first choice to install! If i protect it by a 30mA RCD then it gives no discrimination within the house - unless i provide a time delay 30mA for the submain but you don't see many of these around

2) Many agricultural installatons i have seen have t&e clipped direct in roofs and piped down on drops - i would just like some advice on rcd protection. If a twin and cable is installed in a roof above the height and out of reach from livestock etc surely this is the same as clipping a cable direct in an open ceiling/open beams of a nineteen hundreds' pub where mechanical protection is not required

i appreciate your answers but would also like some practical advice - thank you

 

[JD6400]

Uv degradation for a start on Q1
Mechanical damage from rodents alone on Q2
I have been in more than my fair share of spit and sawdust back water establishments in my younger days as a young farmer , but even those ( I hope ) were generally rodent free !uke:
I would have thought an S type 30mA rcd sort of defeats the object of installing it in the first place !? Hence their rarity ?

 

[UNG]

1)no i would not say twin and earth is the best for an overhead - but this is existing and may have been installed correctly at the time of its regs. I have worked alot on National Trust properties and you find these alot. Perhaps you just work new installations mostly? Done many condition reports containing overhead cables - Advised by NIC inspectors - they are acceptable , providing they are supported correctly and of a suitable height etc . i just was unsure on rcd protection of them? Obviously swa would be first choice to install! If i protect it by a 30mA RCD then it gives no discrimination within the house - unless i provide a time delay 30mA for the submain but you don't see many of these around

2) Many agricultural installatons i have seen have t&e clipped direct in roofs and piped down on drops - i would just like some advice on rcd protection. If a twin and cable is installed in a roof above the height and out of reach from livestock etc surely this is the same as clipping a cable direct in an open ceiling/open beams of a nineteen hundreds' pub where mechanical protection is not required

i appreciate your answers but would also like some practical advice - thank you

If you have done a lot of work on installations of this type how come the question of RCD's has only just become an issue

 

[D Skelton]

Because he is yet another charlatan masquerading as an electrician.

vde-electrician, is your first name Walter by any chance?

:banghead: