What are Safety Grounds?
A temporary connection between de-energized and isolated voltage conductors and the earth for the purpose of safe hand contact. This is typically a large highly conductive copper cable with some form of clapping device at each end. Proper installation of these grounds will enable workers to perform their work duties with maximum safety.
How to Size Safety Grounds
Protective grounding equipment shall be capable of conducting the maximum fault current that could flow at the point of grounding for the time necessary to clear the fault. Protective grounding equipment shall have an ampacity greater than or equal to that of No. 2 AWG copper.
Impedance. Temporary protective grounds shall have an impedance low enough so that they do not delay the operation of protective devices in case of accidental energizing of the lines or equipment.
In grounding applications where a single protective ground cable assembly does not have the necessary withstand current rating, or would require an unacceptably large conductor, identical ground cables may be connected in parallel. To account for unequal current division between parallel grounds, derating multipliers should be applied as follows. For example, two parallel No. 2/0 AWG copper cables, each rated 27,000 amperes for 15 cycles would have a combined rating of 27,000 x 1.8 = 48,600 amperes (instead of 54,000).
Cable lengths add impedance to the grounding system. The goal is to have the lowest impedance to improve the speed response of the OCPD and limit the duration and energy of the fault. An improperly sized grounding system would act as a fuse and when compromised would fail violently potentially causing injury and property damage. No longer than 20’
Inspection, Cleaning and Testing
Ground sets should be inspected and the contact surface areas cleaned before each use. The inspection of temporary safety grounds should include checking for loose connections, contamination and corrosion, and any damage to the clamps, ferrules, or cable.
- Cracked or broken ferrules and clamps,
- Exposed broken strands
- Cut/mashed/kinked or flattened cable
- Cable jacket damage (cracked, swollen or soft spots)
- Cracks or damage to clamps and ferrules
- Check clamps to smooth operation through entire range
- Check torque of pin type terminals and jam nuts on threaded ferrules
- Look for dirt, grease, oil, oxidation on current carrying surfaces
- Any other obvious conditions that would adversely affect performance
If any of these conditions are found in the safety groundT, the grounding jumper assembly shall be permanently marked, tagged or destroyed to prevent reuse or repaired.
As some damage and corrosion in the connections cannot always visible in the regular inspection, periodic electrical low resistance testing should be performed to ensure the ground set’s resistance is within satisfactory levels per industry standards. ASTM F2249 provides details on test methods and pass/fail resistance values for personal protective ground sets.
Also refer to NFPA 70E 120.3.A, OSHA 1926.962, and IEEE 1048 for additional information.
Procedures for Installing Grounds
Step 1: De-energize the line in accordance with procedures.
Use a documented LOTO procedure to be certain that the circuit or equipment has been de-energized and isolated from all sources of hazardous energy. Temporary protective grounds should be placed to create an equipotential zone in the working area near as possible to the work location but at a safe enough distance to limit the physical risk in case a fault causes the grounds to move violently.
Step 2: Test the circuit for voltage.
Don’t assume that the circuit has been de-energized just because it’s been turned off. Other sources of energy, such as induction from nearby circuits, can result in lethal shocks and other injuries.
It is required to perform a 3-point test (live-dead-live) with a sensitive voltage testing devices to verify a zero voltage state. A 3-point test consists of testing the voltage tester on a known energized source to verify it is working properly (Live).
Then, test the circuit on which work is to be performed (Dead).
Finally, test the voltage tester on the same energized source as was used in Test No.1 to verify the tester is still working properly (Live).
Always wear the appropriate levels of shock and arc-flash hazard PPE when applying grounds. Available for purchase from JM Test.
Step 3: Clean all connections.
Extra resistance caused by corrosion and dirt should be eliminated to maintain an extremely low resistance to ground, otherwise single point grounding will be ineffective.
Step 4: Apply ground-end clamps first, and remove them last.
This assures there is no time during installation in which the operator could become the lowest-resistance ground path. Mechanical connections should be strong enough to withstand forces generated by electromagnetic induction.
Step 5: Conductor-end clamps must be applied and disconnected by hot sticks of adequate rating and length.
When not to use Grounding Jumpers
Grounding Jumpers are not intended to protect personal and should not be used for protection In the event of Lightning. Also, for currents exceeding 50,000 amperes, symmetrical Extreme electromechanical separation forces are developed in ground cables. Mechanical failure of the ground cable assembly is likely.
