maximum measured Zs
- Thread starter skeets
- Start date
Sponsored Links
Skeets, if you have measured the Zs then you do not have to calculate it.
A calculation of Zs might be Ze + R1+R2 = Zs.
Example:
1) the R1+R2 of a lighting circuit might be 1.5 ohms (DC resistance)
2 the measured external Ze might be 0.35 ohms (impedance)
3) the Zs would be 0.35 + 1.5 = 1.85 ohms.
If you actually measured the Zs at the end of the lighting circuit then you may well see a slightly different value for Zs for two reasons:
1) there may be parallel paths in the earth path (the main reason)
2) measuring R1 + R2 is done with a DC measurement which may not included reactive components seen at 50Hz AC though the difference is likely to be minimal.
I hope I have understood and answered your questions reasonably well.
A calculation of Zs might be Ze + R1+R2 = Zs.
Example:
1) the R1+R2 of a lighting circuit might be 1.5 ohms (DC resistance)
2 the measured external Ze might be 0.35 ohms (impedance)
3) the Zs would be 0.35 + 1.5 = 1.85 ohms.
If you actually measured the Zs at the end of the lighting circuit then you may well see a slightly different value for Zs for two reasons:
1) there may be parallel paths in the earth path (the main reason)
2) measuring R1 + R2 is done with a DC measurement which may not included reactive components seen at 50Hz AC though the difference is likely to be minimal.
I hope I have understood and answered your questions reasonably well.
Adding to the above... from GN3 the reason that Ze is required to be measured is two fold:
To verify that there is an earth connection.
to verify that the Ze value is equal to or less than the value determined by the designer and used in the design calculations.
Don't forget to test polarity at the origin of the installation - those meter guys sometimes swop the live and neutral tails .... just to test you
Ze is measured using an earth fault loop impedance tester at the origin of the installation. This is a live test.....and I will assume you have the appropriate tester , nulled the test leads and it is working correctly.
Ensure that everyone in the property is made aware that the test is taking place.
Then ensure that the main switch at the Consumer Unit is open.
Remove the suppliers earth connection (to ensure that there are no parallel paths) from the Consumer Unit. Then test between the suppliers incoming live and earth connection. This is your external loop impedance Ze. (note: some testers require you to also connect to the suppliers neutral as well).
Replace the suppliers earth connection. I would normally take the Prosepctive Fault Current at this stage.
For R1+R2 (lighting as described above) ensure that there are no accessories in the circuits (lamps, dimmer switches, motors etc) and switches are closed. This is a dead test so make sure the main switch is open.
At the consumer unit provide a temporary link between the test circuit and the earth bar. (Don't forget to remove it at the end of the test)
Then using a low resistance ohmmeter measure between the line and earth at each point on the circuit - recording the highest reading, which should be the last point on the circuit. You can also test for polarity at this point by opening the switch.
Add Ze to R1+R2 to give you Zs.
Use Zs to check the max earth loop impedance for the MCB with the appropriate tables in the BRB
As stated the measured value of Zs is likely to be less than the Zs=Ze +(R1+R2) value - You can use either.
To verify that there is an earth connection.
to verify that the Ze value is equal to or less than the value determined by the designer and used in the design calculations.
Don't forget to test polarity at the origin of the installation - those meter guys sometimes swop the live and neutral tails .... just to test you
Ze is measured using an earth fault loop impedance tester at the origin of the installation. This is a live test.....and I will assume you have the appropriate tester , nulled the test leads and it is working correctly.
Ensure that everyone in the property is made aware that the test is taking place.
Then ensure that the main switch at the Consumer Unit is open.
Remove the suppliers earth connection (to ensure that there are no parallel paths) from the Consumer Unit. Then test between the suppliers incoming live and earth connection. This is your external loop impedance Ze. (note: some testers require you to also connect to the suppliers neutral as well).
Replace the suppliers earth connection. I would normally take the Prosepctive Fault Current at this stage.
For R1+R2 (lighting as described above) ensure that there are no accessories in the circuits (lamps, dimmer switches, motors etc) and switches are closed. This is a dead test so make sure the main switch is open.
At the consumer unit provide a temporary link between the test circuit and the earth bar. (Don't forget to remove it at the end of the test)
Then using a low resistance ohmmeter measure between the line and earth at each point on the circuit - recording the highest reading, which should be the last point on the circuit. You can also test for polarity at this point by opening the switch.
Add Ze to R1+R2 to give you Zs.
Use Zs to check the max earth loop impedance for the MCB with the appropriate tables in the BRB
As stated the measured value of Zs is likely to be less than the Zs=Ze +(R1+R2) value - You can use either.
As stated above
Zs=Ze+(r1+r2)
Ze being the earth loop impedances at the supply side and
r1 being the resistance of the line conductor, consumer side.
r2 being the resistance of the cpc, consumer side.
When taking a reading of Zs the results should be compared to the values given in Tables 41.2, 41.3 or 41.4. of BS7671:2008 pages 48 & 49
A rule of thumb calculation is made, by multiplying the table value by 0.8. (as appendix 14, BS7671:2008)
The measured value should be lower than corrected rule of thumb value for compliance.
Zs=Ze+(r1+r2)
Ze being the earth loop impedances at the supply side and
r1 being the resistance of the line conductor, consumer side.
r2 being the resistance of the cpc, consumer side.
When taking a reading of Zs the results should be compared to the values given in Tables 41.2, 41.3 or 41.4. of BS7671:2008 pages 48 & 49
A rule of thumb calculation is made, by multiplying the table value by 0.8. (as appendix 14, BS7671:2008)
The measured value should be lower than corrected rule of thumb value for compliance.
Sponsored Links
Or are you wanting to know how to calculate the maximum measured permissable Zs for a circuit?
The regs give the maximum Zs for a cable at it's operating temperature, for PVC this is at 70°C.
When the cable is cold it's resistance will be lower hence the need to compensate when measuring the Zs.
As an example, a circuit fed from a B6 MCB will require 30A to disconnect within 0.4s (from BS7671). This gives a maximum tabulated Zs of 230/30= 7.66ohms at a conductor operating temperature of 70°C.
This figure needs to be corrected for a conductor temperature when doing the measurement, the OSG and GN3 both use 10°C in their measurements.
The simplified temperature coefficient of copper is 0.004 per degrees centigrade.
This can be plugged into the equation for working out the resistance, 1+(0.004 x temp change), 1+(0.004x60) This gives a factor of 1.24.
You can now use this to work out the max measured Zs by using the tabulated Zs and dividing it by 1.24 (which roughly equates to 80% of the tabulated value.)
In the above example , 7.66/1.24=6.18 ohms which is the value you would get if you looked it up in the OSG or GN3
The regs give the maximum Zs for a cable at it's operating temperature, for PVC this is at 70°C.
When the cable is cold it's resistance will be lower hence the need to compensate when measuring the Zs.
As an example, a circuit fed from a B6 MCB will require 30A to disconnect within 0.4s (from BS7671). This gives a maximum tabulated Zs of 230/30= 7.66ohms at a conductor operating temperature of 70°C.
This figure needs to be corrected for a conductor temperature when doing the measurement, the OSG and GN3 both use 10°C in their measurements.
The simplified temperature coefficient of copper is 0.004 per degrees centigrade.
This can be plugged into the equation for working out the resistance, 1+(0.004 x temp change), 1+(0.004x60) This gives a factor of 1.24.
You can now use this to work out the max measured Zs by using the tabulated Zs and dividing it by 1.24 (which roughly equates to 80% of the tabulated value.)
In the above example , 7.66/1.24=6.18 ohms which is the value you would get if you looked it up in the OSG or GN3
If we take that example we can see if it would be compliant to BS7671
Lets say the circuit is being protected by a B6 device.
and the calculated reading is the same as the measured reading.
So we have a Zs of 1.85 ohms
We look up B6 type breaker this can be found in table 41.3
the table value is 7.67 ohms, this is then multiplied by 0.8 (rule of thumb)
giving the corrected value of 6.14 ohms.
As the measured value is 1.85 ohms and lower than the corrected table value of 6.14 ohms, the Zs of this circuit is compliant and disconnection times will be met.
Looks like spark123 has also offered and example using the division of 1.24, this is also acceptable, but it will give you slighty higher corrected value.
Also the corrected values can be found in the OSG tables 2A, 2B, 2C, 2D appendix 2 pages 100-103.
But I stuck with the rule of thumb method of the multiple 0.8 as this is what was required when I was taking my exams.
Hi not trying to be picky but as the second test should only be done on a dead circuit. Riveralt you say for the live test ZE main switch should be open you then say for the dead test main switch to be open too..
Could be a little sparky.
Could be a little sparky.
Adding to the above... from GN3 the reason that Ze is required to be measured is two fold:
To verify that there is an earth connection.
to verify that the Ze value is equal to or less than the value determined by the designer and used in the design calculations.
Don't forget to test polarity at the origin of the installation - those meter guys sometimes swop the live and neutral tails .... just to test you
Ze is measured using an earth fault loop impedance tester at the origin of the installation. This is a live test.....and I will assume you have the appropriate tester , nulled the test leads and it is working correctly.
Ensure that everyone in the property is made aware that the test is taking place.
Then ensure that the main switch at the Consumer Unit is open.
Remove the suppliers earth connection (to ensure that there are no parallel paths) from the Consumer Unit. Then test between the suppliers incoming live and earth connection. This is your external loop impedance Ze. (note: some testers require you to also connect to the suppliers neutral as well).
Replace the suppliers earth connection. I would normally take the Prosepctive Fault Current at this stage.
For R1+R2 (lighting as described above) ensure that there are no accessories in the circuits (lamps, dimmer switches, motors etc) and switches are closed. This is a dead test so make sure the main switch is open.
At the consumer unit provide a temporary link between the test circuit and the earth bar. (Don't forget to remove it at the end of the test)
Then using a low resistance ohmmeter measure between the line and earth at each point on the circuit - recording the highest reading, which should be the last point on the circuit. You can also test for polarity at this point by opening the switch.
Add Ze to R1+R2 to give you Zs.
Use Zs to check the max earth loop impedance for the MCB with the appropriate tables in the BRB
As stated the measured value of Zs is likely to be less than the Zs=Ze +(R1+R2) value - You can use either.
Hmm not sure I understand you
Consumer Unit main switch open (off) for both tests. Conduct live Ze test on the incoming (suppliers )tails to the CU and the suppliers earth connection or alternatively and if fitted at the isolator switch.
Conduct the dead test on the CU circuits after the open main switch. (should have added make sure the circuit breaker are off as well)
I think you're probably being a bit unfair to riveralt here, and I thnink I agree with what he wrote - namely that the main switch should be open for both the tests he mentioned.
The second test is straightforward. It is a dead test, so the main switch obviously needs to be well and truly open.
The first test is a live test, but only involves the (disconnected) earth connection. From the point of view of the test, I can't see that it makes any difference whether the main switch is open or closed (provided one takes one's L for the test from the supply side of the main switch!). However, since the test involves disconnecting the earth from the installation, it seems wise to open the main switch for the duration of the test (until the earth is reconnected) so that no potential hazard exists around the premises due to the absent earth.
That's how I see it, anyway.
Kind Regards, John.
No sorry its a been a long day. I just didnt want someone just changing the switch position because they thought it needed to be in the opposite position for a dead test to a live test.
But as i said picky,
Ill get my coat
But as i said picky,
Ill get my coat
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
