9

LTC1535

Figure 11. Detecting Wiring Faults

Y

A

B

Z

GND2

LTC1535

TWISTED-PAIR

NETWORK

2

EQUIPMENT SAFETY GROUND

EARTH GROUND

2

2

FLOATING RS485 COMMON

*

IRC WCR1206

** IRC WCR1210

*** PANASONIC ECQ-U2A103MV

B1

CN2R (JKL)

1535 F11

R1*

470k

R2*

470k

R3**

100k

R4**

100k

C1***

10nF

2

High Voltage Considerations

The LTC1535 eliminates ground loops on data communi-

cation lines. However, such isolation can bring potentially

dangerous voltages onto the circuit board. An example

would be accidental faulting to 117V AC at some point on

the cable which is then conducted to the PC board.

Figure 11 shows how to detect and warn the user or

installer that a voltage fault condition exists on the twisted

pair or its shield. A small (3.2mm) glow lamp is connected

between GND2 (the isolated ground) and the equipment’s

safety “earth” ground. If a potential of more than 75V AC

is present on the twisted pair or shield, B1 will light,

indicating a wiring fault. Resistors R3 and R4 are used to

ballast the current in B1. Two resistors are necessary

because they can only stand off 200V each, as well as for

power dissipation. As shown, the circuit can withstand a

direct fault to a 440V 3

∅ system.

Other problems introduced by floating the twisted pair

include the collection of static charge on the twisted pair,

its shield and the attached circuitry. Resistors R1 and R2

provide a path to shunt static charge safely to ground.

Again, two resisitors are necessary to withstand high

voltage faults. Electrostatic spikes and transients can be

limited by the addition of capacitor C1 and discharged

through R1–R4.

APPLICATIO S I FOR ATIO