Upgrading 4mm main earth conductor if 16mm does not fit

[sidekick]

An old wylex one way csu with 60A fuse and 16mm tails has 4mm main earth only.
The csu supplies another csu in 10mm t/e and the second csu has max demand of 55A.
The main earth needs to be upgraded but 16mm will not fit into the csu earth bar.
What is the best way to upgrade in this case???

 

[PC Electrics]

What about 2 10mm in two terminals?

 

[telectrix]

or 3 6mm in 3

 

[IQ Electrical]

An old wylex one way csu with 60A fuse and 16mm tails has 4mm main earth only.
The csu supplies another csu in 10mm t/e and the second csu has max demand of 55A.
The main earth needs to be upgraded but 16mm will not fit into the csu earth bar.
What is the best way to upgrade in this case???

Unless you have a TN-C-S system then the best way to upgrade is by way of the adiabatic equation to size the main earthing conductor.

 

[SPARTYKUS]

Sorry to appear belligerant but are you absolutely sure it will not fit.

Have had this discussion on another forum about 16mm2 green and yellow fitting in an old Wylex earth terminal. It will fit (in an old wylex board) but it has to be cut perfectly with no distortion to the cut end whatsoever. As in it fits perfectly .....

But you may be talking about a smaller board and consequently a smaller earth bar?

 

[malcolmsanford]

Chances are if your Ze is over 0.15 ohms then that 4 mm will be just about right, so any value above it will be fine.

If you really feel the need to fit a 16mm then you could "Y" the conductor in the CU and fit into 2 connections if there are enough available.

 

[sidekick]

Thanks for all the advice so far.
The board in question is actually a one way switched fuse with only two earth terminals on the bar. 16MM will definately not fit.
The 4mm in one and the outgoing earth from the 10mm T/E in the other.
Have "y" connected the 16mm and got half into first but only one strand will fit with the outgoing conductor for t/e which leaves 2 of the 6 not fixed.
With the 4mm earth in place Ze at 0.35 and is TNS system. 16mm may not be a must but could do with a refresher on the adiabatic equation to size the main earthing conductor if anyone could oblige.

 

[IQ Electrical]

Have a look at this very good post on the adiabatic: http://www.electriciansforums.net/i...ons/28128-example-use-adiabatic-equation.html

 

[oldtimer]

If the DB is metal then crimp the cable into a lug and bolt it to the unit

 

[sidekick]

If the DB is metal then crimp the cable into a lug and bolt it to the unit

Thanks for that oldtimer but not metal I'm afraid.

 

[malcolmsanford]

If you hadn't disconnected your 4mm earth then you would not get a proper reading for your Ze. So for your let's just say 0.35 value which if you disconnected the earth will be more as you will not be getting parallel paths

S = Min CSA of CPC required in mm^2 to satisfy thermal constraints
I = Uo / Ze in Amps
t = Operating time of the protective device in seconds
k = a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor material

So it's 230/0.35= 658 rounded up now sq that to = 432964

I'm going to take it you have a BS 1361 Fuse and if you go to appendix 3 fig 3.1 pg 244 in the BS 7671 you will see that for any of the sizes 200, 000 plus amps will acitvate in 0.1 seconds

So the next part will be 432964 x 0.1 = 43296.4 which you then sq root = 208.1 rounded up in this case.

As I'm sure you are using copper then the K will be from IMO table 54.2 but to err on caution take it from table 54.3 pg 129 and so it will be

208.1/115 = 1.81 rounded up to 1.81 mm cable, if you took the 54.s table then it would be 208.1/143 = 1.56mm cable.

As regulation 543.1.1 tells us that the minimum earthing conductor can not be less than 2.5 protected and 4mm un-protected I would say your exisitng 4 mm is fine

 

[sidekick]

[/QUOTE]
I'm going to take it you have a BS 1361 Fuse and if you go to appendix 3 fig 3.1 pg 244 in the BS 7671 you will see that for any of the sizes 200, 000 plus amps will acitvate in 0.1 seconds [/QUOTE]

Thanks for the working calcs. Coorect with copper and 1361 fuse.
Not sure where the 200,000 comming from on pg 244.
4MM just seemed a little small as an alternative to the calc in fig 54.7 is S=<16 EARTH TO BE S (where S is 16mm line)
Will double chack that Ze of 0.35 and the disconnected earth.

 

[malcolmsanford]

The graph as 2 axis left is time,seconds and bottom is PC rms Amps if you look at the left one time, and look at 0.1 seconds, you will see that every rating of the fuse starts at the 0.1 part of the graph. Now look at the grid insert on the page and 0.1 secs, As your fault current is 432,964 it is well above the 880amps for a 60amp type, 1100amps for a 80amp type and 1800amps for a 100amp, so I said as your over 200,000 amps, in fact twice over, it will trip within the 0.1s

The table is basically a guide if you don't wish to calculate as to reg 543.1.3

 

[Octopus]

An old wylex one way csu with 60A fuse and 16mm tails has 4mm main earth only.
The csu supplies another csu in 10mm t/e and the second csu has max demand of 55A.
The main earth needs to be upgraded but 16mm will not fit into the csu earth bar.
What is the best way to upgrade in this case???

Should you be replacing the 10mm T&E to the 2nd CU too? Strikes me its a bit on the small side!

 

[sidekick]

Is there a similar calc to confirm if tails are correct size.
Installation has three DB'S one already described with 16mm tails, second with 6mm t/e to a 16A switch fuse and the third with 10mm t/e to 45A switch fuse. 1 and 3 feed consumer units but 2 only feeds a singe socket.

 

[sidekick]

Should you be replacing the 10mm T&E to the 2nd CU too? Strikes me its a bit on the small side!

Good point. sub main supplied by this 10mm is for 11 lights and 5 sockets plus imersion heater then 10mm tails from incommer to another board for 1 unused FCU and 1 socket. Reckon on 55A total. Table 4D5 ref metod c = 64A max

 

[malcolmsanford]

Ib= Design current of the circuit

In=Rated current of your protective devive

Iz = the current capacity of the cable

Then your volt drop

All this is in appendix 4 Page 253 of the Regs.

 

[sidekick]

Ib= Design current of the circuit

In=Rated current of your protective devive

Iz = the current capacity of the cable

Then your volt drop

All this is in appendix 4 Page 253 of the Regs.

Sorry, maybe being a bit thick here. I understand that for circuit design from DB but for tails from meter to DB is the calc the same.
eg if 1g socket supplied on 2.5 t/e from 16A fuse on 1 WAY DB all ok, but how do they get to 6mm t/e from meter to 1 WAY DB

Assuming main fuse 80A should the 6mm t/e and the 10mm t/e tails both be changed to 16mm

 

[Engineer54]

Are they not teaching the importance of the ''Adiabatic Equation'' these days at collage??

It was instilled in us at the company training school from a very early stage... These days even after being shown working examples, you still have this wonderment and disbelief, and a total reliance on tables in the BRB!!!...

 

[IQ Electrical]

Are they not teaching the importance of the ''Adiabatic Equation'' these days at collage??

It was instilled in us at the company training school from a very early stage... These days even after being shown working examples, you still have this wonderment and disbelief, and a total reliance on tables in the BRB!!!...

I hope you're not generalising again?

And sadly, the answer to the question is no, it does not seem to be covered in college anymore, I'd say the majority of guys haven't even heard of it, never mind being able to apply it (forum members excepted)

I know for a fact from teaching 2391-10 revision courses, there hasn't been a 2391-10 adiabatic question in the last 10 years-pretty pathetic really!