4.4 3-wire loop measurement (Z LOOP). CHAUVIN ARNOUX C.A 6456

4.4

WIRE LOOP MEASUREMENT (Z LOOP)

4.4.1

DESCRIPTION OF THE FUNCTION

In a TT network, measurement of loop impedance LPE is a rapid and practical way of checking a earth resistance without planting auxiliary rods.

In this case, the measurement includes the earthing resistance of the power supply transformer of the installation and the resistance of the distribution cables. It is therefore a high earth measurement, and any error is on the safe side.

In TT and TN networks, this function can also be used to check and size the protective systems in place by a rapid and easy measurement of the loop impedances between L and PE, L and N, and N and PE. This function can also be used to calculate the corresponding short-circuit currents (sizing of fuses and circuit breakers).

Note that the instrument allows measurement of loop impedance LPE behind 30mA circuit-breakers without causing them to trip (principle patented by Chauvin Arnoux).

In an IT network, use the instrument's "ZLINE" function.

The measurement principle is the same as for an earth measurement in a live condition.

When the TEST key is pressed, the instrument :

- checks that the amplitude and frequency of the voltages present are correct,

- measures the voltage between the TEST key and terminal PE,

- interchanges L and N internally if the 2 conductors are reversed in the socket,

- generates a current (high or low according to the user's selection) between terminals L and PE,

- measures loop impedances Z

Note: the measurement of Z

LN

LN

, Z

LPE

, et Z

NPE

.

does not trip the earth fault breakers even with high currents.

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4.4.2

PREPARATION FOR THE MEASUREMENT (CONNECTION))

=> If necessary, in SET-UP mode:

- set threshold voltage U

L

(see § 3.2),

- the type of compensation of the measuring cables (see § 3.3))

- set the alarm threshold Z

L

or R

L

- possibly, select a value of U

REF

(see § 4.2.2)

to be used for the calculation of the short-circuit current :

PUSH btn

2 nd

>

88.88

direction of alarm (> or<)

88.88

location of decimal point

88.88

MORE x3 setting of the values of each of the 4 digits, selected by successive presses on the

keys

PUSH btn

- possibly, select a value of U

REF

to be used for the calculation of the short-circuit current:

PUSH btn

MORE x4

U ref

MEAS

I k selection of voltage :

MEAS/230/400/127 or MEAS/220/380/110

MORE x3

PUSH btn

- the current generated for the measurement in low current mode (see § 4.2.2),

- set the number of measurements to be counted to smooth the measurement (see § 3.2)

=> Set the switch to the ZLOOP position,

=> Activate the alarm by pressing the ALARM key ,

=> Connect the line power plug or the 3 separate cables to the installation to be tested,

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=> Compensate the measuring cables (see § 3.3),

2 nd 5

=> high ( ) for a greater accuracy:

- if no tripping of a earth fault breaker is foreseen (measurement made upstream of the RCDs),

- if the earth fault breaker concerned is short-circuited for greater accuracy,

=> low ( ) otherwise for a rapid check

The connection diagrams are the same as for earth measurements in a live condition, but without the voltage or current probe (see § 4.2.2).

4.4.3

MEASUREMENT PROCEDURE

The instrument first checks the resistance of the rod and measures the voltage between PE and earth, then measures voltages U

LN

, U

LPE

, U

NPE

.

If these values are correct, pressing the TEST key starts the measurement: as soon as the result is available, it is displayed.

Note: for measurements on three-phase systems, the loop impedance must be measured between each phase conductor, the neutral conductor, and the protective conductor.

4.4.4

MEASUREMENT RESULTS

After the measurement, the measured values and complementary results can be consulted using the and keys.

(The quantities accessible before the measurement is made are described above, in § 4.1.4.)

Initial display

Initial display

R

LPE

Z

LPE

MORE

(1 st press)

L

LPE

Z

LPE

MORE

(2 nd press)

U

REF

I

KLPE

MORE

(3 nd press)

Hz

U

LPE

MORE

(4 nd press)

----

Z

L ALARM

MORE

(5 nd press)

R

∆

PE

U

L

(1 st press)

R

LN

Z

LN

L

Z

LN

LN

U

I

REF

KLN

Hz

U

LN

Z

----

L ALARM

R

U

∆ L

L

(2 nd press)

R

NPE

Z

NPE

L

Z

NPE

NPE

I

U

REF

KNPE

Hz

U

NPE

Z

----

L ALARM

R

U

L

N

Note : In non-trip mode the inductive component is not measurable due to the weak test current. The values

L

LPE

, L

NPE

, Z

NPE

, are not displayed (display shows - - - -). The value L

LPE

is displayed in the L

LN

field .

4.4.5

CHARACTERISTICS

4.4.5.1 Measurement ranges and accuracy

Particular reference conditions :

- nominal voltage of the installation = 90 to 550V,

- nominal frequency of use = 15.3 to 65Hz,

- inductive part < 0.1 x the resistive part of the impedance measured.

The characteristics of loop measurements with 3 wires, with ( ) or without tripping ( ) are the same as those of earth measurements in a live condition, with or without tripping, respectively: see § 4.3.5.1.

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