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Posted By Topic: New distribution board

mariner77
May 30 2017 04:32

Adding a new Distribution board even it is only a 4 way garage to new Main switch board need MEN link not connected?
I seen most of Distribution boards with MEN link at Main Switch board and Also at Distribution board .
As my understanding Now MEN link at DBs not permitted.
   

gregmcc
May 30 2017 05:42

depends on how you are going to feed the sub board, are your running a PEC? if so then no MEN link. otherwise, put in and earth stake and fit a MEN link to the sub board
   

mariner77
May 30 2017 06:29

Running 2c+E Tps to sub Board with MEN link at Main Sw Board and no MEN link at Sub Board.

if someone can clearify why Link at Sub board is not prermitted while at Main Switchboard there is MEN link already.

My understanding on this was Neutral is to be Earthed at each subBoard to ensure prompt operation of protection devices.
??

   

gregwires
May 30 2017 08:30

Hello,

have a think about the return path for normal operation for your sub board. You're putting the E & N of your feed in parallel if there is an link at each end.
   

AlecK
May 30 2017 08:39

There are 2 permitted ways of supplying a DSB.
Main way is with the submain including an earth conductor; in which case NO link at DB.

The other way is using the Submain N as also being the principal earthing system for the DB - same as the mains of a house. But unless the DB is in an "outbuilding"; this PEN submain option is NOT permitted [8.3.8.1(a)].

Either way; the issue is that load current musty NOT flow in the PEC. If you run A, N, & E, and put in a N-E link; then return load current will flow in both N & E conductors.

Yes you'll find a lot of older submains with both PECs in their submains and N-E links. They were installed according to old rules.


   

pluto
May 30 2017 08:47

mariner77 May 30 2017 06:29
Your comments
Running 2c+E Tps to sub Board with MEN link at Main Sw Board and no MEN link at Sub Board.

if someone can clearify why Link at Sub board is not prermitted while at Main Switchboard there is MEN link already.

My understanding on this was Neutral is to be Earthed at each subBoard to ensure prompt operation of protection devices.

My comments
The neutral to earth link in switch boards other the mains switchboards was a requirement of the Electrical Wiring Regulations prior to 1993.

The main reason the practice was discontinued was to remove load current from flowing the sub main protective earth conductors from the main switch board and the distribution switch board.

With a N to E link in the distribution AND the main switch boards, the sub main neutral and protective earth conductors are connected in parallel and thus the load current flows in both the N and E conductors.
   

RonProffit
May 30 2017 09:25

Just to add a bit to Pluto's answer. All conductors have some resistance, as the cable sizes are the same size and length we can assume the neutral & earth wires have the same resistance and will share the neutral current equally, but as you are now passing this through the res of rthe earth wire you will develop a voltage across that earth wire, meaning that an earth connected to the sub board will be at a different potential to the main earth.
   

AlecK
May 30 2017 09:32

Actually in most cases the PEC is smaller; so the current isn't split evenly.
But yes having current in the PEC causes earth potential rise, which means chance of touch voltage between equipment / structures connected to various switchboard and also between earthed parts and items connected to actual ground.
   

pluto
May 30 2017 10:28

If there is an electronic data distribution system through the electrical installation, the differences in touch voltage between items of data equipment connected to different switch boards, often results in data signalling errors between devices due the electronic equipment logic 0 (earth) connections.

This is because in most equipment Logic 0 and protective earth are the same.

When there is high 3rd, 5th, 7th and up to around 30th harmonic currents from non-linear power supplies the touch voltages can be a real problem.

Some times it is necessary to use isolation devices in the data equipment interfaces, if the differences in touch voltage is high, or better still use a fibre optic signal interface.


   

mariner77
May 31 2017 10:11

So what should be appropiate action when come across old distribution/Sub Board
With MEN link.

Remove link or leave it as existing work?
   

AlecK
May 31 2017 10:23

leave as existing. Just in case there are other design deficiencies that rely on the PEC being in parallel.

But good idea to leave a note at swbd that DB (x) has a N-E link, to save some poor sod wondering why his IR reading is flat, or he seems to have huge "leakage" in the submain.