Bullrush charts

[NozSpark]

I've just been thinking..... scary isn't it.. 

Is there a formula anywhere to get accurate disconnection times from the bulrush charts

 

[kerching]

I thought about this many many years ago.

then I thought..........why bother!

so i got a piece of acetate sheet from an OHP, marked two lines in black perm marker at 90 degree to each other.Works well,eough for my meagre needs

 

[Blue Duck]

surely you have enough spare regs books floating about you could draw on?

OHP... that that's me back a couple of decades!

 

[NozSpark]

The thing is I don't want to draw on them...

Having a formula for this would allow me to write a spread sheet formula to work out the Adiabatic with only 2 or 3 input fields

... just thinking

 

[Rob_the_rich]

The thing is I don't want to draw on them...

Having a formula for this would allow me to write a spread sheet formula to work out the Adiabatic with only 2 or 3 input fields

... just thinking

I have done this before with 3 input fields (in yellow), does this help? you still have to look up the disconnection time from the tables.

View attachment Adiabatic equation.xlsx

 

[NozSpark]

I was hoping that I could do something similar,, but with the disconnection times being auto filled by selection of device type IYKWIM

TBH I've checked the "worst" cable and that passes the adiabatic so the rest will be fine

 

[Rob_the_rich]

I don't think there would be a formula (lines are curves on a logarithmic scale), what you could do is create tables for each of the OCPDs listed (and BS1361 from previous bs7671) using the acetate from OAP, then "lookup" those tables, you would still have 3 inputs, Zs or Ze, k and ocpd.

Did I say OAP? I meant OHP.

 

[NozSpark]

There must be a formula,,, I doubt that the IET just draw them

 

[steptoe]

Ye. Probably, maybe,

Who would know,? The NICEIC just make things up as they go along, :slap

Maybe the IET do too judging by the turnover speed of the regs,

Anyway, as said, it's like decibels, it's not a linear thing, 

I've never actually had an instance where it's that close tbh, so never thought about it much. 

 

[Rob_the_rich]

I think they get the graphs from manufacturers' data, and join the dots. the manufs would certainly have tested them at a range of fault currents.

To make the graphs into tables would take a looong time for each ocpd, just looking them up seems easiest.

 

[Blue Duck]

Surely different manufacturers would have different curves? They could be an average of all data collected but I doubt it.

was watching bigclive last night and he was waffling on about ring mains and all that the IET do is make new and exciting ways to test them.

Are they just making it ALL up?

 

[Rob_the_rich]

oh, page 317 of byb says "in all of these cases time/current characteristics are based on the slowest operating times for compliance with the regulations and have been used as the basis for determining the limiting values of earth fault loop impedance prescribed in Chapter 41"

So it seems the iet took all the manufs test results and used the slowest times recorded which still met the standards.

 

[Manator]

I am a little confused as to what you want to be honest, that's my ignorance. The curves describe let through currents and I have always used the PFC as the main calc along with t =0.1 and 1.45, or 3600 seconds.

So if you have a PFC of 2000 this is (2000X2000) x 0.01

I²t x 0.1

Please ignore my ignorance, just trying to get my head around what you mean.

 

[sprocketflup]

Them graphs are a total PITA to read and no mistake, I find a clear plastic set square useful, some amount of estimation still needs to be used tho

 

[Manator]

Them graphs are a total PITA to read and no mistake, I find a clear plastic set square useful, some amount of estimation still needs to be used tho

I must be missing a trick here, for years I have just calculated. Could someone enlighten me on how you use the described set square and the two 90 degree markers. I think I am learning a new trick here, and here am I thinking I know everything. :slap :coat

 

[Rob_the_rich]

I am a little confused as to what you want to be honest, that's my ignorance. The curves describe let through currents and I have always used the PFC as the main calc along with t =0.1 and 1.45, or 3600 seconds.

So if you have a PFC of 2000 this is (2000X2000) x 0.01

I²t x 0.1

Please ignore my ignorance, just trying to get my head around what you mean.

The curves show the (slowest) operating time of an ocpd plotted against the current that causes it to operate.

To work out the diameter of an earthing conductor using the adiabatic equation, you need the maximum time a conductor can safely pass a current, as described in 434.5.2 (That formula is rearranged in 543.1.3 to give the minimum csa). This time is obtained from the curves for a particular magnitude of fault current (which is derived by dividing the voltage by the measured loop impedance)

You know all this though.

 

[Manator]

Yes, but how do you use the set square and the two 90 degree markings to work out (ballpark) figures.

From the OP I am guessing that the question is about fuse links? I always calculate the cable size according to the fuse characteristics and use the above calculation method in doing so. I have never used a set square or any other method and I am intrigued as to how it works.

 

[Rob_the_rich]

Yes, but how do you use the set square and the two 90 degree markings to work out (ballpark) figures.

From the OP I am guessing that the question is about fuse links? I always calculate the cable size according to the fuse characteristics and use the above calculation method in doing so. I have never used a set square or any other method and I am intrigued as to how it works.

Ze=0.35 fault current=230/0.35=632A

eg fig 3A1 bs88-3 sys C 80A

go up from 632A until hit 80A curve

go across to read time 0.6 secs

without a set square or lines, your eyes go all fuzzy.

Edit; my eyes went funny, should be ~1.4 secs, the corner of a sheet of paper works too, but the last decimal point is guesswork as sprocketflup said.

 

[NozSpark]

That's exactly what I'm after,,, but by calculation with formula rather than using the tables so that I only have to enter the protective device and Zs,,, and it'll churn out a CPC size

 

[NozSpark]

I had 9 x 6.0/2.5 sub mains fed off 5 x B40 60898's (in pairs) to verify

I then realised that I only had to do the calculation on the "worst" one and it'd verify them all