From the City & Guild Chief examiners reports it appears that many get calcultations of Zs, Ze, R1 and R2 wrong. Remember folks I have not yet taken the course or the 2391 exam.
Zs is the sum of Ze, R1 and R2 seems simple enough. Zs= Ze + (R1+R2).
Ze is the external impedance. Although you can calculate the value, it is more normal that the value is obtained by measurment using a suitable instrument or by enquiry.
R1 is the resistance of the phase conducter
R2 is the resistance of the CPC or circuit protective conducter.
In practical test situations the value of R1 + R2 is obtained in one test so could in effect be considered as one value say Rt. So the equation would become Zs = Ze + Rt.
I just hope that in the 2391 exam you are not expected to know the values allowed in BS 7671.
So why Z and R?
Z is the impedance and applies to AC circuits.
R is resistance and applies to DC circuits.
Friday, 22 June 2007
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1 comments:
When the value of R1 + R2 is measured then this will be lower than that obtained in practice and to adjust to the actual the 3/4 rule is used so to take into consideration the temperature. Only like measuring the resistance of an incandescent lamp using a multimeter then taking the current taken when lit. The multimeter shows a lot lower resistance.
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