Can leakage voltage affect 7550/7650 digital inputs?

Issue
Can leakage voltage affect 7550/7650 digital inputs?

Product Line
ION7550, ION7650

Environment
Digital inputs on ION7550 and ION7650 meters connected to external triggering devices.

Resolution
Goals and Symptoms
In certain cases, a device connected to and triggering the digital inputs on an ION7650 or ION7550 can have a leakage voltage. A leakage voltage occurs when a device produces a small amount of voltage across its terminals when it is open (ideally, this should be 0 volts).

When the device closes the circuit, the digital input registers a trigger. When the device opens the circuit, the digital input should not register a trigger. However, if there is leakage voltage when the triggering device is open, testing is required to determine if the leakage is enough to falsely trigger the input.

Facts and Changes

• The internal resistance between Scom and S8 or S7 is 230 kΩ on meters manufactured after September 2009.
• On older 7550/7650 meters (before September 2009), the internal resistance was 40 kΩ.
• The minimum current required for the meter’s digital input to register a pulse is 50 microamperes (µA).

Causes and Fixes
If the leakage voltage across the digital input connection produces a current of 50 µA or more, the input may falsely register as closed.
To resolve this:

• Calculate the leakage current using Ohm’s law:
  Current (A) = Leakage Voltage / Internal Resistance
• If the current is ≥ 50 µA, modify the circuit to reduce the leakage current (e.g., by adding a resistor or using an isolation relay).

The minimum amount of current required for the meter at the digital inputs to register a pulse is 50 micro A.

Hence with this information, figure out if a leakage voltage across the digital input connection is enough to produce a 50 micro ampere current. If the leakage produces a 50 micro ampere current or more work out the circuit to reduce the current.