A ground-check relay decides from loop current, not from the number of conductors connected to its terminal. When two GC wires are paralleled onto one relay and one grounded 5.6 V Zener diode, either wire can carry the test current. Opening one wire therefore leaves a valid current path through the other. The monitored quantity stays inside the relay's healthy window, so the relay cannot identify the failed cable.
Common fixes that leave a blind spot
Paralleling both GC wires at the motor and panel appears to add redundancy, but it changes the monitoring objective. The circuit then proves that at least one parallel path remains; it does not prove that both paths remain intact.
| Proposed arrangement | What the relay can prove | Failure left undetected |
|---|---|---|
Two GC wires in parallel, one relay, one Zener diode |
At least one parallel path can conduct the supervisory current | Either individual GC path can open while the relay remains healthy |
One selected GC wire, one relay, one Zener diode |
The selected cable's check circuit is intact | The second cable is not supervised |
| Two Zener diodes connected to one relay input | Only the combined impedance presented to that input | The relay still lacks two independent channel states; parallel devices may also change the expected current |
| Two independently attributable channels | Each cable's check loop separately | No individual open should be masked if cross-paths have been eliminated |
Using only one cable is valid only when the requirement is to confirm one monitored grounding path to the motor. It cannot be recorded or described as proof that both installed cables are intact. Likewise, adding a second end device does not create another measurement channel when both devices feed the same relay input.
Current, heat, and response time
The number that matters is the supervisory current through the relay, the selected GC conductor, the end-of-line Zener diode, and the grounding return defined by the relay design. The relay classifies that current or the corresponding loop impedance as healthy, open, shorted, or faulted. Two parallel conductors are electrically equivalent to redundant current paths, so loss of one branch may produce little or no change at the relay.
The 5.6 V value is the nominal Zener voltage selected for this installation. Compatibility still depends on the relay's required end-of-line device, polarity, current window, and wiring method. Read those values from the relay connection diagram and technical data rather than treating any 5.6 V diode as interchangeable.
| Quantity | Limit or decision | Where to read or calculate it |
|---|---|---|
| Zener voltage | 5.6 V nominal in the proposed installation | Diode marking and datasheet; compare with the relay manual |
| Supervisory current | Must fall inside the relay's healthy window | Relay diagnostics or technical data |
| Zener thermal load | Calculate with measured loop current, then compare with the diode rating and enclosure temperature derating | |
| Loop resistance | Must leave sufficient current at the complete installed cable length | Measure end to end and compare with the relay's permitted loop resistance |
| Operate and reset time | A simulated fault must persist long enough for the relay to recognize it | Relay timing settings, status indication, and manual |
This is heat, not logic: diode dissipation rises with current. A voltage match alone does not establish an adequate diode power rating. The relay's measuring current, diode tolerance, ambient temperature, and enclosure temperature determine the required margin.
Monitoring objective and channel architecture
Choose the architecture from the required claim. If the protective function requires both cables to have individually supervised ground-check paths, use two independent relay channels and an end device for each channel. Each channel must remain attributable to one cable from the panel terminal through its GC conductor to the motor termination.
If the requirement is only to confirm that the motor has one monitored grounding path, one relay and one Zener diode can supervise one selected cable. The second cable then remains outside the ground-check diagnostic coverage, even if its grounding conductor is connected normally.
Parallel grounding conductors and common bonded metal can create return paths around an intended cable-specific test. Two relays and two Zener diodes provide individual indication only when the relay manufacturer's approved topology prevents one cable's grounding path from satisfying the other channel. Trace the complete current loop, including bonded enclosures and grounding conductors, before calling the channels independent.
Independent-channel wiring procedure
- Write the functional requirement as either “at least one monitored path” or “each cable monitored independently.” Base the relay count on that statement.
- Obtain the exact relay wiring diagram. Confirm that a 5.6 V Zener diode is the specified termination and identify its required polarity, allowable loop resistance, measuring current, and response behavior.
- For individual cable supervision, allocate one relay channel, one
GCconductor, and one end device to each cable. Keep the two pilot circuits separated at terminal blocks and junction points. - Map the return path for each channel. Check whether shared grounding terminals, bonded motor metal, cable armor, or parallel equipment-grounding conductors allow current to return through the other cable.
- Mount each end device at the remote termination point specified by the relay design. A panel-mounted termination may prove panel wiring while leaving the field cable outside the supervised loop.
- Record conductor identity, end-device polarity, measured loop resistance, normal relay indication, and any configurable timing or sensitivity setting.
De-energize and secure the motor circuit before opening a ground-check conductor or disturbing a protective grounding connection. Use an approved test link or manufacturer-defined simulation when removing an actual grounding path could create an injury or equipment hazard.
Fault-injection verification
A continuity reading at installation proves only the state measured at that moment. Acceptance testing must show that every failure the design claims to detect changes the correct relay state.
- Place both circuits in their normal connected condition and record each channel's healthy indication and diagnostic value, if available.
- Open the test link for cable A's
GCpath. Channel A must leave the healthy state while channel B remains healthy. - Restore cable A, verify reset, and repeat the test for cable B.
- Simulate loss of the remote end device using the relay manufacturer's test method. Only the associated channel should respond.
- Test for cross-path masking by opening one channel while all common bonds and the other cable remain connected. A channel that stays healthy is not independently supervising that cable.
- Hold each simulation beyond the configured or specified response time, then verify both operation and reset behavior.
For a one-relay parallel arrangement, repeat the single-branch open test deliberately. If the relay remains healthy with either branch open, label the function accurately: it detects loss of all usable parallel paths, not loss of either individual cable.
Recurring acceptance pitfalls
A handheld meter's diode mode can identify a junction and polarity, but it may not drive the diode into its 5.6 V breakdown region. Verify the end device with a current-limited method appropriate to its datasheet or through the relay's documented diagnostic procedure.
Disconnect sensitive relay and end-device electronics before insulation-resistance testing when their manuals require isolation. An insulation tester can apply a voltage far above the ground-check measuring circuit's normal level.
Document the distinction between protective grounding and supervision. The ground-check circuit reports the condition represented by its measured loop; it does not make an unsupervised cable visible, and a healthy indication does not prove two independent paths when parallel conductors or bonded returns can mask a single open.
FAQ
How do I monitor both motor cable ground-check wires independently?
Use two independently attributable relay channels with one GC conductor and one approved end device per channel. Fault-test each cable separately while the other cable and all normal bonds remain connected.
How do I tell whether parallel ground-check wires are acceptable?
Parallel wiring is acceptable only when the required claim is that at least one monitored path remains. Opening either individual branch while the relay stays healthy demonstrates that the arrangement cannot supervise both cables separately.
How do I verify that the 5.6 V Zener diode is suitable?
Compare its nominal voltage, polarity, power rating, and temperature derating with the relay's specified end device and measuring current. Calculate diode load as , using the measured or documented Zener current.
How do I know when to stop testing and contact official support?
Stop when the relay diagram does not define the 5.6 V termination, either channel remains healthy during its individual open test, or common grounding paths prevent an attributable loop. Keep the circuit de-energized if testing would disturb protective grounding, and provide the relay manufacturer’s official support channel with the wiring drawing, measured loop values, relay indications, and fault-injection results.