calculating max zs values please

[dean7828]

hi chaps, could some bright spark help me out here, i have loads of periodics comming up and am getting fed up of looking up zs values for new and old MCBs, i kind of know the basic maths behind the values but could someone clarify how to work it out on site so

max zs values for type 1,2,3 mcbs for the older type and a,b,c for the newer type. im sure someone could help and would be most greatfull as for some reason we never covered this in the 2391???

thanks in advance.

 

[johnboy6083]

firstly you need to knowe the instataneous tripping current for your MCB. (Ipf)

B type= 3-5 x In
C type= 5-10 x In
D type= 10-20x In (bsen60898, also allows upto 50x In)

type 1 = 2.7- 4 x In
type 2= 4- 7 x In
type 3= 7-10 x In
type 4= 10- 50 x In

your max Zs is calculated by Zs= Uo/Ipf

 

[malcolmsanford]

This is how I do it but unfortunately you need the Ia of the protection device and therefore the regs

Zs values are indicated in BS 7671-2008 in tables 41.1 to 41.5.

The following is the calculation to determine Zs values without the tables


Zs = Uo
Ia

Where Zs Loop impedance in ohms
Uo Phase voltage in volts
Ia Current giving automatic operation of protective device in amps,
In the stated time 0.1 to 5 secs

Note Appendix 3 of the BS 7671-2008 gives tables from pg 243



Example

230v supply with a 20amp HRC BS 88 fuse with a disconnection time of 0.4 secs.


Zs = 230 = 1.77 ohms.
130




Table 41.2 Shows this to be 1.77 ohms

 

[dean7828]

spot on thanks guys.

 

[telectrix]

alternatively, there is a sticky thread entitled " useful information for BS7671:2008 Electrical Form Filling". all the more common max. Zs values are in there, as well as loads of other useful stuff. download, print, laminate. stick it in your kit.

 

[MarkieSparkie]

Hi guys, Please excuse me ramping off a slight tangent. Does anybody have any insight into why the figures in the 230V<Uo<=400V column of Table 41.5 are at odds with all the other columns in so much as they don't appear to be derived directly from R[SUB]A[/SUB]xI[SUB]delta n[/SUB]<=50V like the other tabulated values. Or is it just the case that it wasn't spotted and amended in the July 2008 corrigendum, and more importantly has it been amended in the BGB (can't check, my copy has been borrowed by a mate who's taken it on holiday for light reading).

 

[ezzzekiel]

typeb would be 5x its rating divided by the voltage

32a tybe b x5 = 160 /230 = 1.44 ohms

 

[Paul.M]

Thanks guys for explaining it in a different way, it makes better sense to me now and very simple to work out on site.

 

[ebow72]

firstly you need to knowe the instataneous tripping current for your MCB. (Ipf)

B type= 3-5 x In
C type= 5-10 x In
D type= 10-20x In (bsen60898, also allows upto 50x In)

type 1 = 2.7- 4 x In
type 2= 4- 7 x In
type 3= 7-10 x In
type 4= 10- 50 x In

your max Zs is calculated by Zs= Uo/Ipf

When doing your calcs for Ipf, do you always use the highest value for instance...
B type= 3-5 x In
C type= 5-10 x In
D type= 10-20x In

Thanks in advance, Ian

 

[ebow72]

When doing your calcs for Ipf, do you always use the highest value for instance...
B type= 3-5 x In
C type= 5-10 x In
D type= 10-20x In

Thanks in advance, Ian

It's all good I've worked it out and all figures are coming up as per the regs, I can't believe how simple of a equation this is.
I've been doing some pass papers for my 17th Edition, but when referencing the regs, it's not that easy to work out the Max ZS Values. I have a chart I have used but can't take it in my exam with me, but now I know this, it's a whole lot simpler to take in and understand.
Thanks for the advice....Ian

 

[spark 68]

Don't forget to multiply your result by 0.8 (80% rule of thumb) to allow for temperature correction to 70 degrees C

 

[ebow72]

Ahh I didn't factor that in, but when I do I don't get a matching figure on the pass papers, I have the chart which shows me the Maximum Permissible Values and the test readings based on 80% of maximum values, so it works fine for that. So I am guessing test book answer is Maximum Permissible Values ( in Ω ) of Loop Impedance (Zs) as permitted by BS7671:2008.
But thank you for the reply spark 68, much appreciated.
Ian

 

[spark 68]

ebow,
Sorry to add confusion, I did not realise you were looking for the 17ed answers,

I thought I was replying to a PIR/2391 question

The values given in BS7671 are tabulated values

 

[Guest123]

Hi guys, Please excuse me ramping off a slight tangent. Does anybody have any insight into why the figures in the 230V<Uo<=400V column of Table 41.5 are at odds with all the other columns in so much as they don't appear to be derived directly from R[SUB]A[/SUB]xI[SUB]delta n[/SUB]<=50V like the other tabulated values. Or is it just the case that it wasn't spotted and amended in the July 2008 corrigendum, and more importantly has it been amended in the BGB (can't check, my copy has been borrowed by a mate who's taken it on holiday for light reading).

Sorry Mark, missed this one,

essentially it's all connected to the disconnection times on a TT system.

This was a reply to the same Q, ages ago........

We're talking about disconnection times on a TT system.

We know that 50 / .03 = 1667ohms safe touch voltage requirement met.

In table 41.1, column 2, nominal voltge up to 230v, are our standard disconnection times of - 0.4s TN and 0.2s s for TT.

Now the time/current characteristics table for RCDs, tells us we can meet those disconnection times with a residual current of about 2x I delta N - (roughly 60mA)

Under fault protection circumstances,

120v (the lowest in the range) / 1667 = 0.071 A which exceeds the 60mA value.

In the next column, your disconnection time drops to 0.07 sec, so, we need a residual current of 5x I delta N to meet this, yet the lowest voltage in the range is only 231v, so...

231v / 1667 = 0.138A....... which does not exceed the 150mA value required for disconnection

Using the calculation - 230 / 1533 = 0.150 A

Column 4, the lowest voltage in the range is 400v, disconnection time 0.04 - again we need 5x I

400v / 1667 = 0.239 A


Hope this helps a bit.​

 

[ebow72]

Sorry guys I was looking for a way of locating max Zs values and came across this thread, does the principle of determining the max Zs not apply now? Yes I'm just going through some pass papers and trying to work out the values from the regs. I have a table I've used previously but can't take it in my exam. Thanks though for the comments guys, I've picked up tons of info on here.