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grounding cables

Grounding Cables

GENERAL

There are 2 types of cables used in the electrical industry: grounding cables and insulated jumper cables.  Grounding cables are used to temporarily connect a phase line to the ground while the phase line is disconnected for servicing.  This helps to keep the repair crew safe.  Insulated jumper cables are used to temporarily tie a hot phase line to a line that you need hot.  These will be discussed in a future article.  Grounding cables can be single lines with a connector at each end or they can be configured with 4 connectors to allow shorting of all 3 phases simultaneously of a 3-phase system.  The intent here is that if someone accidentally turns the breaker back on this cable (and it’s short) will cause the circuit to trip before a repair crewman gets hurt.  These cables are typically made using 4/0 stranded cable. This cable can handle many hundreds of amps and the stranding makes it flexible enough to make the connections easier.  At every end, there is a connector to allow secure clamping to an uninsulated portion of the disconnected “hot” line or the ground line.  While the cables have insulation, it is generally not intended to protect against a high voltage so much as to provide mechanical abrasion resistance for the copper cable itself.

CALIBRATION FREQUENCY

While ASTM F2249 does not recommend a specific time interval for testing these types of cables, OSHA and other agencies recommend annual testing.

CALIBRATION PROCEDURE

We follow the practices listed in ASTM F2449 (Standard Specification for In-Service Test Methods for Temporary Ground Jumper Assemblies Used on De-Energized Electric Power Lines and Equipment) when doing grounding jumper testing.  These cables are first cleaned.  The insulation on the cables is wiped down with an appropriate cleaning agent to remove most grease and dirt. This will allow the cable part of the assembly to be examined for damage and to read the wire gauge printed on the insulation.  Then connectors are cleaned with a wire brush. This will remove dirt and oxidation from the jaws of the connector.  Next, the length of the cable is measured.  This length (to the nearest inch), along with the wire gauge and the ambient temperature, will determine the total resistance of the “cable” part of the jumper assembly.  There are charts which give this information.  Now add to this the resistance of the connectors and any “Y” connections. This is typically about 0.16mΩ per connection. If the cable is a single cable, then it has one connection at each end or a total of 2*0.16mΩ added to the cable length resistance calculation.  If the cable is a 3-phase cable then there are a total of 4 connections in the total tested length (or 0.64mΩ worth of connector resistance).  The cable to be tested is securely connected to the tester (this is a device capable of supplying up to 500A of current). The cable should be laid out in a specific pattern or shape, depending upon AC or DC testing and the test equipment manufacturer’s recommendations.  The manufacturer also will specify the current setting to use for a particular gauge of cable. This current is applied to the jumper cable, and the resistance is read from the test equipment.  This is compared to the calculated resistance of the jumper.  If there is more than a 5% deviation then there is probably a poor connection, and the jumper will need to be completely disassembled, cleaned, and re-assembled to eliminate the poor connections.  Once the cable assembly is re-build it will need to be tested again.  If the resistance is still too high the jumper assembly should be rejected and marked as such.  Otherwise, the cable is good and is so marked with the date tested and the expected re-test date.  The cable is removed from the tester, carefully rolled up, and placed in the carrying/storage bag.

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