GFIV1.2 Ground Fault Monitor Circuit Board

 

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Circuit Description

The DTEC Ltd. GFIV1.2 ground fault monitoring board places a high impedance (~2M2 ohms) voltage source of +25.6 VDC, with respect to chassis ground, onto eight channels. It then measures the drop across sensing resistors. These (1M2 single phase 1M8 3 phase) sensing resistors are found either installed on the GFIV1.2 termination board or installed in an epoxy block close to the high voltage source. If a ground fault condition is developing this +25.6 vdc level decreases. If +25.6 VDC is measured with respect to chassis ground on the monitored channel there is no ground fault. As the this voltage drops, a corresponding decrease in the ground fault resistance level is sensed and an increasing analog output voltage for that channel is produced. If the ground fault level becomes less than an equivalent of 500K ohms to ground the digital alarm is raised on the board.

There are 5 sections to the circuit board:

 

  1. Power Supply
    The 2 - DC-DC converters receive a logic +15 vdc and output +/- 15 vdc. These work in series to create a +60 vdc circuit and a single +15 vdc with respect to chassis ground. Minimum current required by the board is 2 amps to startup and nominally 500ma running. Typical load in a no ground fault condition is 170mA (normal operation).
  2. Voltage Comparitor
    This section compares the trip level (set by Pot 11 and measured on test point 1) to the measured sense input level. Pot 11 is set to 6.00 VDC or 500 KOhms.
  3. Digital Output
    This section triggers the digital output to be enabled. This level is either active high (+5 or +12 vdc) or active low (0 vdc). The active high/low is selectable by Jumper 1 (JP1). No matter what the state of JP1 the LEDs on the board only illuminate if the ground fault trip level is reached.
  4. Analog Output/Isolator
    This section outputs 1 voltage as selected by the bit select inputs. ( C12, C14, and A14). The output is with reference to power supply ground and should range from +0.00 to +5.00 vdc.
  5. Bit Select Input
    This is the section which selects the channel that will be routed through the analog isolator. The following is a table for the bit select levels and the corresponding channel to be routed to the analog output.

Channel #

Bit A (lsb)

Bit B

Bit C

Bit D (msb)

1

1

1

1

1

2

0

1

1

1

3

1

0

1

1

4

0

0

1

1

5

1

1

0

1

6

0

1

0

1

7

1

0

0

1

8

0

0

0

1

9

1

1

1

0

 

 

 

 

 

Note:lsb : Least Significant Bit
msb : Most Significant Bit
0 : Logic Ground
1 : Logic +15 vdc

 

 

 

Board Setup

The ground fault board is set up at the factory to measure ground faults ranging from 100 M ohms to 0 ohms. For further adjustments or re-tuning follow these instructions.

No foreign wiring (system interconnections to sense lines) should be connected to the board only the essential wiring.

Connections for the board

Terminal Decription

Edge Term board

C2 39 Chassis Ground

C4 38 Logic Ground

C10 1 Logic +12 vdc

C12 2 Bit Select 1 (lsb)

C14 3 Bit Select 2

A14 4 Bit Select 3 (msb)

C16 5 Digital Output Alarm For Channel 1

C18 6 Digital Output Alarm For Channel 2

C20 7 Digital Output Alarm For Channel 3

C22 8 Digital Output Alarm For Channel 4

A16 9 Digital Output Alarm For Channel 5

A18 10 Digital Output Alarm For Channel 6

A20 11 Digital Output Alarm For Channel 7

A22 12 Digital Output Alarm For Channel 8

C26 36 Channel 1 Input Sense Lines

35

C28 34 Channel 2 Input Sense Lines

33

C30 32 Channel 3 Input Sense Lines

31

C32 30 Channel 4 Input Sense Lines

29

A26 28 Channel 5 Input Sense Lines

27

A28 26 Channel 6 Input Sense Lines

25

A30 24 Channel 7 Input Sense Lines

23

A32 22 Channel 8 Input Sense Lines

21

 

Test points used on the board

Input logic voltage level point and ground point found in the rear left corner.

Chassis ground point found near LED 8

TP1 found near power LED9 and C18

TP3 found near C19 and P10

Setup

Tune Pot 11 (P11) so the voltage at Test Point 1 (TP1) is +6.00 vdc with respect to chassis ground. This calibrates the digital output trip level to activate at ~500K ohms. Tune Pots 1 through 8 (P1 to P8) so the voltage at the junction of the diode and capacitor is +10.00 vdc with respect to chassis ground. For example to set up channel 1: - place the “+” ve probe of multi-meter on the junction of D1 (diode 1) and C1 (capacitor 1) and the “-“ ve on the chassis ground. - tune P1 until the multi-meter reads +10.00 vdc. E.g. To setup channel 4: Remove U8. U8 cannot output a negative voltage which is required on setup of the board. U8 will be replaced later. Place a full ground fault (two 1M2 resistors in series from chassis ground and the input) on channel #1 (C26). If using the terminal board connect chassis ground to both terminal #36 and terminal #35. Tune Pot 10 so that the analog output at pin 3 of U8 is 5.00 VDC. Remove the ground fault on C26 and tune Pot 9 so that the voltage out is -0.83VDC. Replace the ground fault on C26 and re-tune Pot 10 continuing to go between the no ground fault condition and ground fault condition tuning Pot 9 and Pot 10 respectively.

           
** Note **
If you are using the DTEC GFM-3 termination board you must tie the 2 input sense together ( 35,36 - 33,34 etc.) so the sense resistors are parallel. If they are already connected to a power supply or a transformer they don’t need to be tied together but you should be wary of trying to setup the board in circuit as ground faults even of a small magnitude will throw off the setup values.

Selectable features:

Jumper 1 JP1 - This selects the digital output from either active high or active low.

Jumper installed is active low. Jumper removed is active high. Jumper 3 JP3 - +5 vdc or +15 vdc - This is a hard wire jumper found in front of P11.