Ra is the sum of the electrode and CPC resistances. Table 41.5 gives you the maximum Zs to satisfy the Ra x In < 50V equation.Correct me if I'm wrong but thought it was Ra not Ze on TT?
thanks guysI've always worked on the principle that a good circuit within the property should comply with the mormal Zs values, adding Ze for the rod to get a total Zs value. However reading the regs suggests anything upto 1553 ohms is sufficient, which never felt right as this a huge heat generating resistance (worse than required to melt showers switches) ergo that last comment always struck me as nonsense, hence I work the way I do.
Just to be 100% accurate - a high current generates heat when passed through a resistor, anything the current passes through is a resistor - even the cable.Sorry to correct you apache but high resistance does not cause heat. heat is caused by the piece of equipment/joint/circuit not being able to handle the high currents associated with LOW resistances. As you stated high resistance eventually causes no current flow thus no trip of rcd's .
I know - that's what I saidSorry Patch, but you have it wrong..The value of the resistance determines the amount of current drawn.
The higher the resistance, the lower the current and vice-versa
eg 1V with 1ohm
I=V/R 1(A)=1(V)/1(ohm)
1V with 0.5ohms
2(A)=1(V)/0.5(ohms)
HEAT IS DEVELOPED BY HIGH CURRENT
Just to be 100% accurate - a high current generates heat when passed through a resistor, anything the current passes through is a resistor - even the cable.The science - when a greater current is flowing there are more electrons been exchanged (or excited) in the same area. The more electrons moving the more electrons collide and it is these collisions that change the electrical energy into thermal energy.
High resistance doen't cause heat in itself - only when current passing through! I know what I mean, but can see how I phrased it badly.
Table 2.3, page 48 Guidance note 3, Insp & Testing.Actually I was told it was 1666 ohms ie 50v/30ma
But it should be less than 200 ohms to account for ???? the weather in Guatamala
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