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20
Operation
Measurement methods and sources of error
The selected DC test voltage (current limited to less than 2 mA DC) is applied to the circuit under test and the
resistance is calculated from measurements of the resulting voltage and current.
Capacitive circuits can take some time to charge. This is displayed as an increasing voltage that takes longer to
reach its maximum than normal.
The reading is stable with a circuit capacitance less than 5 µF.
Ground Fault Current Interrupt (GFCI) testing
The MFT70 can perform a ramp test on a 6 mA GFCI which introduces a current to ensure it trips under fault
conditions.
This GFCI test meets the requirements of GFCI Type A as defined in the ANSI/UL 943, CSA C22.2 No. 144.1 and
ANCE NMX-J-520.
Function button options
Short press - 0º / 180º starts the test on a positive or negative half cycle.
Hold 2s press – AC / A / S / B type
Note:
When testing GFCIs the test mode should be left in AC
which is set as default. The other modes
are not relevant for GFCI testing. If another mode is selected unintentionally, e.g. A
, S
S
or B , then
the mode button
can be held down for 2 seconds to change the mode. Repeat this until the
A symbol
is
displayed at the bottom of the display.
Ramp test
The GFCI trip current is measured by applying a test current and increasing this every 300 ms. When the GFCI trips,
the current flowing is recorded and displayed in mA if it is close to the rated current of the GFCI i.e. between 5-6
mA, to a resolution of 100 µA. Alternatively the display will show a ‘trp’ message to showing the GFCI has tripped.
Making a measurement
1. Select the GFCI
GFCI
option on the right hand rotary switch.
2. Select the RAMP
test on the left hand range rotary switch.
3. Connect the instrument Phase (L1) and Ground (L2) terminals to the GFCI phase and ground terminals (or to
the phase and ground of the circuit the GFCI is protecting). Use either the separate leads or mains plug leads.
RCD
g
L
N
E
MCB
L
N
E
MCB
g
