Encyclopedia of Grounding (CA09040E)

If R 1 represents a line worker and R 2 the personal protective jumper, the equation becomes:

I

(R JUMPER ) x I AVAILABLE

Eq. 5a

I MAN =

(R MAN + R JUMPER )

I 2

I 1

Resistances in parallel circuits can be reduced to a single, equivalent value for use in calculations. This is done by:

Parallel Circuit Fig. 5-5

1

= 1 + 1 + 1 ...… 1

Eq. 6a

R TOTAL R 1 R 2 R 3

R LAST

A simplified form of Equation 6a when dealing with only two resistances is found by algebraically rearranging the equation. Remember R 1 and/or R 2 could be the sum of a series of resistances.

In this case, again the lamps have equal resistance and the current divides equally between the two paths. If there are unequal resistances, the current divides in inverse proportion to their resistances. That is, the lower the resistance of the path, the more current goes through that path. This is the foundation principle of personal protective grounding, placing a very low resistance jumper in parallel with a much higher resistance worker. Figure5-6 shows theparallel circuitwith the lamps replaced by the electrical symbol for resistance. Equation 5 shows the calculations for this circuit.

R TOTAL = (R 1 x R 2 ) / (R 1 + R 2 )

Eq. 6b

A key point in parallel circuits is that some current will flow through every possible path. The current magnitude ineachpathwill dependupon the resis tance of each path. The only means of completely eliminating current flow is to eliminate the path. In any circuit a voltage drop is developed only if current flows through the resistive element. And, the larger the resistance, the larger the voltage drop, as shown in Fig. 5-7.

I

I

I

1

2

R

R 2

1

Parallel Circuit Fig. 5-6

For example:

Fig. 5-7

R 2

I 1 =

x I TOTAL

Eq. 5

(R 1 + R 2 )

(Remember, current divides in inverse proportion to the total resistance)

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5-5