Manual Gossen Metrawatt PROFITEST MXTRA (page 26 of 96) (English)

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26 GMC-I Messtechnik GmbH

8 Testing of Breaking Requirements

Overcurrent Protective Devices,

Measurement of Loop Impedance and

Determination of Short-Circuit Current

(functions Z

L-PE

and I

)

Testing of overcurrent protective devices includes visual inspec-

tion and measurement. Use the PROFITEST MASTER or SECULIFE IP

to perform measurements.

Measuring Method

Loop impedance Z

L-PE

is measured and short-circuit current I

ascertained in order to determine if the breaking requirements for

protective devices have been fulfilled.

Loop impedance is the resistance within the current loop (utility

station – phase conductor – protective conductor) when a short-

circuit to an exposed conductive part occurs (conductive connec-

tion between phase conductor and protective conductor). Short-

circuit current magnitude is determined by the loop impedance

value. Short-circuit current I

may not fall below a predetermined

value set forth by DIN VDE 0100, so that reliable breaking of the

protective device (fuse, automatic circuit breaker) is assured.

Thus the measured loop impedance value must be less than the

maximum allowable value.

Tables containing allowable display values for loop impedance

and minimum short-circuit current display values for ampere rat-

ings for various fuses and circuit breakers can be found in the

help texts and in section 21 beginning of page 88. Maximum

device error in accordance with VDE 0413 has been taken into

consideration in these tables. See also section 8.2.

In order to measure loop impedance Z

L-PE

, the instrument uses a

test current of 3.7 to 7 A (60 to 550 V) depending on line voltage

and line frequency. At 16 Hz, the test has a duration of no more

than 1200 ms.

If dangerous contact voltage occurs during measurement

(> 50 V), safety shut-down occurs.

The test instrument calculates short-circuit current I

based on

measured loop impedance

L-PE

and line voltage. Short-circuit

current calculation is made with reference to nominal line voltage

for line voltages which lie within the nominal ranges for 120 V,

230 V and 400 V systems. If line voltage does not lie within these

nominal ranges, the instrument calculates short-circuit current I

based on prevailing line voltage and measured loop impedance

L-PE

Measuring Method with Suppression of RCD Tripping

The PROFITEST MXTRA and SECULIFE IP provides users with the

opportunity of measuring loop impedance within systems which

are equipped with RCCBs.

The test instrument

generates a direct

current to this end,

which saturates the

RCCB’s magnetic

circuit.

The test instrument

then superimposes

a measuring cur-

rent which only

demonstrates half-

waves of like polar-

ity. The RCCB is no

longer capable of

detecting this mea-

suring current, and

is consequently not tripped during measurement.

A four conductor measuring cable is used between the instru-

ment and the test plug. Cable and measuring adapter resistance

is automatically compensated for during measurement and does

not effect measurement results.

Select Measuring Function

Connection:

Schuko / 3-Pole Adapter

Connection:

2-Pole Adapter

Note

Loop impedance should be measured for each electrical

circuit at the farthest point, in order to ascertain maxi-

mum loop impedance for the system.

Note

Bias Magnetization

Only AC measurements can be performed with the 2-

pole adapter. Suppression of RCD tripping by means of

bias magnetization with direct current is only possible via

a country-specific plug insert, e.g. SCHUKO, or the 3-

pole adapter (neutral conductor necessary).

Note

Observe national regulations, e.g. the necessity of con-

ducting measurements without regard for RCCBs in Aus-

tria.

3-Phase Connections

Measurement of loop impedance to earth must be performed at

all three phase conductors (L1, L2, and L3) for the testing of over-

current protective devices at three phase outlets.

Start

t1 t3

Measurement

Operation

RCD Disabled!

/mA

Suppression of RCCB tripping for RCCBs

which are sensitive to pulsating current

L-PE

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