TNS DNO earth Braid

[davesparks]

It seems petty I know, but in a technical trade accuracy is important

 

[newtothis]

So
Ipf = 250/0.29
Ipf = 879A
I'll round it up to 900A for simplicity
We also need the disconnection time from appendix 3, which gives us 0.6seconds for 900A
And a value of K of 143 from table 54.2

So for the adiabatic equation we have values of
I = 900A
t = 0.6S
k = 143Hopefully this will make sense!

Yes it makes sense, I'm trying to figure out how you got the value for t.
(1) I know I'm supposed to look up those graphs - but I don't know which one is applicable.
Is it the main protective overcurrent device, or if you don't mind naming the page in appendix three.

Thanks for your patience

 

[davesparks]

Yes it makes sense, I'm trying to figure out how you got the value for t.
(1) I know I'm supposed to look up those graphs - but I don't know which one is applicable.
Is it the main protective overcurrent device, or if you don't mind naming the page in appendix three.

Thanks for your patience

Value for t is, as you say, obtained from the graphs in the appendix.
Figure 3A3(a) on page 322 is where I got it from.

 

[newtothis]

Great thanks

 

[newtothis]

TBH I was giving the adiabatic equation (AE)a wide berth - I’m well pleased to be on the verge of understanding thanks to the help on this forum.

The first time I looked at these graphs, I just saw some kind of confusing spiders web.
I’ve sussed how the scales work. I’m posting it up here to check I’m using the graphs correctly.

Starting with my 900A PFC on the horizontal axix
Going up to crossing line
Then across to the vertical axis (time)

I’m getting 0.2 rather than 0.6 for “t”, is my methodology wrong ?
Plugging the t value into the (AE) I get 2.8

Taking this further, If I was to check the required CSA of CPC on a final circuit I’d use the Zs rather than the ZE I’ve used in this example ?

 

[davesparks]

TBH I was giving the adiabatic equation (AE)a wide berth - I’m well pleased to be on the verge of understanding thanks to the help on this forum.

The first time I looked at these graphs, I just saw some kind of confusing spiders web.
I’ve sussed how the scales work. I’m posting it up here to check I’m using the graphs correctly.
View attachment 29903
Starting with my 900A PFC on the horizontal axix
Going up to crossing line
Then across to the vertical axis (time)

I’m getting 0.2 rather than 0.6 for “t”, is my methodology wrong ?
Plugging the t value into the (AE) I get 2.8

Taking this further, If I was to check the required CSA of CPC on a final circuit I’d use the Zs rather than the ZE I’ve used in this example ?

Yes you are using the graph correctly, but that doesn't look like the same graph as I was looking at.
I'll have a look in my regs book when I get home and get back to you.

 

[Leesparkykent]

TBH I was giving the adiabatic equation (AE)a wide berth - I’m well pleased to be on the verge of understanding thanks to the help on this forum.

The first time I looked at these graphs, I just saw some kind of confusing spiders web.
I’ve sussed how the scales work. I’m posting it up here to check I’m using the graphs correctly.
View attachment 29903
Starting with my 900A PFC on the horizontal axix
Going up to crossing line
Then across to the vertical axis (time)

I’m getting 0.2 rather than 0.6 for “t”, is my methodology wrong ?
Plugging the t value into the (AE) I get 2.8

Taking this further, If I was to check the required CSA of CPC on a final circuit I’d use the Zs rather than the ZE I’ve used in this example ?

Is that the graph for bs88-3? I don't have my BYB on me. But Davesparks must of been lookin at the graph for bs88-2 I think.

 

[newtothis]

Thanks I've checked it out, and think know what table Davesparks used. 3a3(b)
I used 3a3(a)
I need to learn up about what graph to use.
I think you experinced sparks can look at the fuse even tho it's not labled up and know what graph to use - I can't.:44:

 

[newtothis]

All that's on the DNO fuse is "lucy oxford", I happen to know it's a 60a fuse.
No way would I be able to suss the BS no of it ? :sad_smile:

 

[Richard Burns]

Lucy house service cut outs (as most of them) are designed to accept BS1361 fuses, because BS1361 has now been withdrawn and replaced with BS88-3:2010, therefore you should look at the graph for BS88-3 (type C) fuses.
However there is also the problem that the standard fuse ratings have changed a bit so a 60A BS1361 standard rating is now a standard rating of 63A in BS88-3, which really does not help much.
Grab the red book and use the BS1361 graphs!
Then you would find the fusing time at about 900A would be about 0.095 s slightly slower than the just off graph possibly 0.07 s of the 63A in BS88-3.

 

[newtothis]

Thanks,
Hmmm who carries all those books LOL, unfortunaley I've only got the green and yellow.
Is there anything (an alternative method) like a table in the OSG that would work.

 

[Leesparkykent]

Thanks,
Hmmm who carries all those books LOL, unfortunaley I've only got the green and yellow.
Is there anything (an alternative method) like a table in the OSG that would work.

......

 

[davesparks]

Based on the above couple of posts I apologise for my above posts which are incorrect. I was basing my calculations on a 100A bs88-2 fuse.

Would somebody please pass me the dunce's hat, I'll be over in the corner.

 

[newtothis]

Leesparkykent - that wouldn't be the relevant graph from the BRB by any chance LOL ?

On a side note have I got a case for asking the DNO to upgrade the cut out ?
I been taught about asking for Ze by enquiry and stuff.
The reality with my DNO UKPN is they're not interested in anything that isn't an earner for them.

 

[newtothis]

Based on the above couple of posts I apologise for my above posts which are incorrect. I was basing my calculations on a 100A bs88-2 fuse.
Would somebody please pass me the dunce's hat, I'll be over in the corner.

No mate, no dunces hat because you've taught me useful stuff I couldn't understand before. It's my responsibility to check, understand and do my own learning, not to be spoon fed, by good people like yourself and others who help.