Earth fault loop impedance

[gazdkw82]

Iv been given the scenario:

6 circuit earth fault loop impedance values were recorded. Each circuit has type B cb.

referring to the maximum values, show by calculation if these values are acceptable.

so I can't do zs =ze+R1/R2 because they've not provided me with ze.

doing some research it looks like it could be this calculation

Voltage/10= 230/10 =23
Devide that by the A rating of circuit = 23/32 (32A as an example) =0.719
Then times by 2 = 0.719x2 = 1.44

is that the correct way to approach this scenario?

Thankyou

 

[hightower]

So you have the Zs readings in front of you? Is it then not just asking you to find the maximum values in the BS7671 tables, then calculate what would be the maximum measured allowable?

So say (BGB so might be different) 32A type B is listed as 1.44 - so then you apply cMin and also the 0.8 rule of thumb:

1.44 * 0.95 * 0.8 = 1.09ohms

So provided your measured Zs for the 32A circuit is less than 1.09ohm you are a-okay

 

[Andy78]

Iv been given the scenario:

6 circuit earth fault loop impedance values were recorded. Each circuit has type B cb.

referring to the maximum values, show by calculation if these values are acceptable.

so I can't do zs =ze+R1/R2 because they've not provided me with ze.

doing some research it looks like it could be this calculation

Voltage/10= 230/10 =23
Devide that by the A rating of circuit = 23/32 (32A as an example) =0.719
Then times by 2 = 0.719x2 = 1.44

is that the correct way to approach this scenario?

Thankyou

The correct, and more simple way, would be to cross reference the maximum values from the regs with the measured values and see if they comply, taking into account temperature correction if needed.

If you want to calculate the max values from the OCPD size, the formulas are listed at the right hand side of the max Zs table

Type B 230x0.95/(5In)
Type C 230x0.95/(10In)
Type D 230x0.95/(20In)

 

[gazdkw82]

Have you not been provided with the Zs Values of each circuit?

If so then you should be referring to BS7671 table 41.3 (for 60898 MCBs), this table will give you the Max Zs for each protective device. Once you have found the value you then need to multiply it by 0.8. 0.8 is the temperature correction factor to take in consideration the temperature of the circuit's cables when tested. You can then compare these Max values with the measured values you have been given, if your measured values come under then it means the results comply and the protective device will trip off in the required times.

I mean no offence and its good that you have the forum to help you with your queries, but have you not been provided with a tutor that you can contact when you need support? I know you are on a Home study course but I thought these providers also give you support.

Thankyou for your help

Yes I can ring and speak to a tutor but in all honesty thisforum is so quick and informative. Sometimes the tutors are not alwaysavailable to take a call and also im not always available to make a callbecause im either at work or have the kids screaming whilst im trying tolisten. This forum really helps because I can take it in better. I don’t wantto annoy anybody though so I apologise if I have or am being a nuisance

 

[gazdkw82]

No you aren't annoying anyone this section of the forum is made to help trainees, it just seems a shame that you pay a lot of money and do not get the support.

One more thing how long have you been studying and have you actually done any practical training? Just to know your level of understanding, because some of these questions are expected to be answered by someone who has a decent understanding of inspection and (live) testing (level 3) and have the practical understanding too.

I started early december so 5 months so far. Iv completed 2 workbooks covering a lot of aspects, basics, distribution, installation, testing and inspecting. These questions are my pre practical revision. Im at the point where I can do 2 weeks of practical. Iv been going back into the books Iv done so far and they do explain how to do the physical testing/inspecting etc but don't really explain the calculations that Iv needed to actually do these pre attendance questions. It's something I will bring up when I go. I'm doing a lot of Internet searching and reference finding to find the answers. I know this course it's ideal but I'm stuck with it tbh and I just want it done so I can move on. I really do appreciate the help

 

[telectrix]

easy way to work out max ZS for a particular MCB is as follows.use the formula R=V/I, (ohms law), where V = 230V and I is as follows:

B type, I = 5In

C type I = 10In

D type I = 20In

Whre In is the rating of the MCB

then apply your 0.8 factor to allow for the fact that measurements are taken at a nominal 20 deg.C.

 

[gazdkw82]

easy way to work out max ZS for a particular MCB is as follows.use the formula R=V/I, (ohms law), where V = 230V and I is as follows:

B type, I = 5In

C type I = 10In

D type I = 20In

Whre In is the rating of the MCB

then apply your 0.8 factor to allow for the fact that measurements are taken at a nominal 20 deg.C.

would that be 5*In for type B?

 

[telectrix]

would that be 5*In for type B?

yep....

 

[Flanders]

easy way to work out max ZS for a particular MCB is as follows.use the formula R=V/I, (ohms law), where V = 230V and I is as follows:

B type, I = 5In

C type I = 10In

D type I = 20In

Whre In is the rating of the MCB

then apply your 0.8 factor to allow for the fact that measurements are taken at a nominal 20 deg.C.

In the byb they have reduced the maximum zs values now so a b32 amp mcb was 1.44 is now 1.37 sorry i forgot the reason why , might be to do with Europe it normally is

 

[Simonslimline]

In the byb they have reduced the maximum zs values now so a b32 amp mcb was 1.44 is now 1.37 sorry i forgot the reason why , might be to do with Europe it normally is

C min factor.

 

[ruston]

You seem to have a good head on you and I hope you complete your training. Keep doing your research and as you have already seen there are some great guys here willing to help. If you do not already know about there is a online company called learninglounge, they have a bank of decent videos and I think they do a student only registration. It would probably be a decent investment for someone like you training from home.

Just to add to that , there are a lot of free learning lounge videos on you tube.
Here is a link to one of the introductions.
https://www.youtube.com/watch?v=QXEuxfXUkd8

 

[gazdkw82]

would that be 5*In for type B?

So In the book I'm given a value for circuit 1 of 0.5, rated at 40A.

The tabulated value is 1.15

Type B is I = 5In

so is this correct??

R=V/I
V=230
I = 5x40 = 200

R = 230/200 = 1.15

1.15*0.8 = 0.92

 

[hightower]

So In the book I'm given a value for circuit 1 of 0.5, rated at 40A.

The tabulated value is 1.15

Type B is I = 5In

so is this correct??

R=V/I
V=230
I = 5x40 = 200

R = 230/200 = 1.15

1.15*0.8 = 0.92

Sorry just glancing as watching hmrc videos but think you've missed cmin too. If you want to check your calculated value it should match with the on site guide as this already has the factors applied.

 

[Andy78]

So In the book I'm given a value for circuit 1 of 0.5, rated at 40A.

The tabulated value is 1.15

Type B is I = 5In

so is this correct??

R=V/I
V=230
I = 5x40 = 200

R = 230/200 = 1.15

1.15*0.8 = 0.92

Cmin factor has been incorporated into the third amendment. The formula should be 230*0.95/(5In). The tabulated value is actually 1.09 to the current edition of the regs.

See my previous post in this thread and table 41.3 of the BYB.

 

[gazdkw82]

Cmin factor has been incorporated into the third amendment. The formula should be 230*0.95/(5In). The tabulated value is actually 1.09 to the current edition of the regs.

See my previous post in this thread and table 41.3 of the BYB.

In my book, that's the tabulated value it gives me