Protecting the tester

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Each of the 100 or more test points in a typical cable tester connects directly to an integrated circuit (IC) used in applying or measuring test signals. Although various means of overload protection can be built into each point, it becomes economically impractical to isolate the test points from more than a few volts higher than the maximum test voltage. Should the operator inadvertently connect a live cable to the tester, severe damage may ensue.

If the overvoltage from the live cable is considerably above the test voltage, breakdown of the affected IC may transmit the overvoltage through the power bus, reaching many interconnected ICs and rendering the circuit board unrepairable. ICs may actually explode leaving a blackened crater on the inside of the case (Figure 1).

Figure 1.  Overvoltages applied from live cables to sensitive tester inputs can result in ICs exploding within instrument cases. To prevent this type of damage, keep power sources away from cable-test benches.

• Do not allow any power sources on the test bench.
• Keep the work area clutter-free and uncrowded.
• Disconnect any unused interface cables from nearby computers or other equipment.
• When testing long cables, ensure that both ends are labeled, and include in the test procedure a positive confirmation that both ends are detached before attaching the cable to the tester.
• Ensure that any batteries that may be attached to the cable, or built into the cable, are disconnected.
• If you test harnesses built into an equipment cabinet, ensure that all conductors and the shield conductor (if tested) are isolated from ground during testing. Ground differential voltage may cause the test to fail or damage the tester.

Another potential source of electrical hazard is static discharge that can enter the test-point terminals. Taking the usual precautions of working on a grounded workbench with dissipative mats, grounding the test equipment, and wearing a wrist strap may not be sufficient to protect the cable tester from static damage. Charge may develop on the insulation of long cables as a result of frictional motion when the operator coils or uncoils the cable while moving it to the test bench. Charge on the insulation can then attract opposite charge on the copper conductors just under the insulation. This in turn forces charge to the cable endpoints where it remains trapped (Figure 2).

Figure 2  a) Friction can develop a negative charge on cable insulation, which attracts a positive charge to the b) copper conductor. That positive charge, in turn, c) drives negative charge toward the ends of the cable, which can result in tester damage, particularly for cables 10 ft long or more.

Preventive measures:

• Work with a humidity level of 60% or higher (although this is not usually possible during winter months).
• Equip the tester with transient suppressor boards in which special high-speed Zener diodes protect each point from transients higher than the test voltage and less than ground.
• Attach a grounding plug to the cable momentarily before connecting it to the test equipment. The grounding plug consists of a mating connector in which all pins are connected together and tied with a single wire to an earth ground.

You also need to protect your cable-harness tester from power-line transients. A 1-s power interruption may disrupt batch testing and cause a loss of log data or batch reports, requiring a supervisor to restore normal operation and possibly requiring the need to repeat a test. Power surges and switching noise risk damage to equipment as well as stored data.

Preventive measures:

• Use a 600-W to 1000-W uninterruptible power supply (UPS). Such a UPS should be sufficient protection for most workstations, and it represents a small fraction of the cost of the equipment it protects. (A UPS typically costs less than $100 for the tester alone, or less than $200 for both the tester and computer.)
• If your facility provides building-wide uninterruptible power, add a surge suppressor to the tester's power input.

The final electrical hazard that you should worry about is debris that can conduct electrical charges. Wire clippings, metal punch-outs, metal dust, or spilled beverages may cause unintentional connections between test points, or they may work their way onto the circuit board and introduce shorts. Coffee and soda are highly conductive when liquid, and they leave conductive residue when dry, creating a difficult repair problem—especially for testers measuring isolation resistance above 1 MÙ.

Preventive measures:

• Do not permit cable assembly or repair in the vicinity of the test equipment.
• Do not permit food or drink at the test station.
• Place a dust cover over the test equipment when it is not in use.
• Avoid situating the test equipment in the same room as grinding or metalworking machines.

A cable tester can also become inoperable as a result of mechanical hazards, such as defective connectors. If the physical characteristics of connectors used on your cables are slightly out of specification, they may deform or in other ways damage the mating connector on your cable tester.

For example, plastic RJ45 modular plugs sometimes have excess unremoved flashing from the mold, or sharp edges, which catch the wire pins on sockets. When unplugged from the tester, the wire pins may hang up on the flashing and become bent upon removal, permanently damaging the socket. Note that the same problem may damage your customer's connectors and you may be held responsible.

Preventive measures:

• Carefully inspect sample parts before committing to a supplier.
• Require a quality certificate from the supplier before accepting incoming parts.

Even high-quality connectors, though, can eventually cause problems. The natural wear caused by the friction of inserting or removing connectors from mating sockets cannot be avoided. Some simple precautions will prevent premature failure and quickly restore equipment to a functioning condition.

Preventive measures:

• Use connector isolators (also known commercially as "connector protectors" or "connector savers"). These small adapters insert between the cable tester and cable connector to absorb the force and wear of repeated insertions. They may be easily unplugged and changed when necessary and are widely available for D subminiature connectors.
• Require that the mating connectors on your test equipment have solid metal pins, not stamped pins, and include gold plating.

Test technicians can also be a source of connector problems. Technicians who apply excess force off-axis from the insertion direction may bend the pins or shell of a mating connector.

Preventive measures:

• Provide proper training and written procedures for test technicians.
• Situate the cable tester, connector boards, or panel so that the force applied during insertion easily aligns with the connector axis.
• Ensure that keying marks or other indicators of orientation appear clearly on the mating connectors.
• Some cables have intentionally plugged holes in female connectors to prevent backward insertion. Expressly mark the technician's work sheet or procedure if hidden orientation keys exist in the test cable.

You also need to take proper care of a tester's ancillary components. Most cable testers use plug-in connector boards to accommodate many different connector styles. When boards are detached from the tester, connectors may be damaged if not properly stored.

Preventive measures:

• Store boards in a rack so the connectors do not make contact with other boards during storage.
• Use a dust cover over large or fragile connectors, or put boards in bubble bags.
• Store filled racks in closed cabinets.

Our dependence on complex systems in manufacturing leaves us vulnerable to process failures that may have expensive consequences. By taking well-thought-out precautions, you may avoid the preventable accident that can extract a heavy cost in time and inconvenience.