1c Appendix C Ground Hook Safety.pdf

Engineering Division Electronic Equipment Build Standard Appendix C, Ground Hook Safety Revision 2

C.1 Introduction

C.1.1 Scope

This policy is written to cover the use of ground hooks on R&D equipment only. A ground hook system places a grounded conductor at the end of the rod between the operator and the electrical hazard. The low impedance ground hook system is not a live line tool and there will not be a voltage potential at the end of the ground hook rod. The integrity of the system will be measured by the performance of the grounding cable.

High Z ground hooks may have a voltage potential at the end of the rod and will be held to a stricter standard than low Z ground hooks. High Z ground hooks are used in equipment with >1000 joules energy storage to limit discharge current.

C.1.2 Work Practices

C.1.2.1 Before use of ground hooks LOTO shall be applied. A LOTO procedure shall be written for every ground hook application.

C.1.2.2 Only LBNL QEWs shall apply ground hooks

C.1.2.3 High Energy Discharge

a) A properly installed ground hook must first be connected to the high Z discharge point and then to a low Z discharge point (less than 0.1 ohms) that is identified by a yellow circular marker.

b) Written procedures shall include the discharge time for the circuit and when it is safe to apply the low Z ground hook(s).

c) If there is not a designated high Z discharge point then a high Z ground hook may be used on a low Z discharge point. Limiting the discharge current to 500A also applies to the use of a high Z ground hook.

i) Alternatively a QEW may perform measurements and then apply a ground hook to equipment after it has discharged to <5J and <50V.

C.1.2.4 Refer to the Electrical Safety Manual (found in PUB 3000, Chapter 8, Electrical Safety Program) for additional work practices.

C.1.3 Definitions

Z: The letter Z is used as shorthand for impedance. In this document, impedance is equivalent to resistance and is measured in ohms.

Low Z Ground Hook: A ground hook with a resistance (cable and ground connection) equal to or less than 0.1 ohms.

High Z Ground Hook: A ground hook with a series resistor to limit the discharge current. The required resistance value should be determined by an electrical engineer or the manufacturer of the equipment.

Hard Ground: A low Z connection measured at less than 0.1 ohms

Soft Ground: A high Z connection through a resistance to limit the discharge current

C.2 Construction Standard

C.2.1 Ground hook specifications

C.2.1.1 Ground hooks systems are connected to the building ground system. The systems engineer must evaluate the connection point for to ensure the integrity of the ground hook system.

C.2.1.2 The cable should be grounding or welding cable, flexible rope lay, soft drawn copper. The individual strands should not be larger than #30 AWG.

C.2.1.3 Grounding cables have flexible elastomer or thermoplastic jackets primarily for mechanical protection of the conductor it covers.

C.2.1.4 Be crimped according to manufacturer instructions. The lugs shall be rated for the cables to be clamped.

C.2.1.5 When purchased, the rod in the ground hook assembly shall meet ASTM F711 specifications or the OSHA equivalent (see 5.b.ii). The same applies to rods used to manipulate equipment within an HV enclosure.

C.2.1.6 The ground hook rod must have a guard or demarcation on the rod that restricts the operator’s hand placement to outside of the restricted approach boundary.

C.2.1.7 Low Z ground hooks shall be connected such that impedance is less than 0.1 ohms to ground. High Z ground hooks shall be connected such that impedance is less than 0.1 between the resistor and the ground connection.

C.2.1.8 In permanent equipment installations the cable conductor size must be at least #2AWG or larger. For temporary or benchtop situations, the conductor must be capable of safely carrying any potential current.

C.2.1.9 There should be a sufficient number of ground hooks to conveniently, and adequately, ground all designated points.

C.2.1.10 The ground hooks should be grounded and stored in the immediate area of the equipment in a manner that ensures they are used.

C.2.1.11 The grounding cable should be visible from end to end for inspection purposes. When this is not possible, the cable which is difficult to see must be strain relieved. If this portion of the grounding cable gets modified, it must be re-tested (see ESM) before use.

C.2.1.12 The approach path to the equipment being grounded should be perpendicular to the equipment and clear of debris or obstructions. The ground hook operator will then always be facing, and have the grounded conductor between them and the potentially energized equipment

C.2.1.13 Low Z ground hooks shall be left in place during the time of safe access.

C.2.1.14 Voltage indicators that are visible from all normal entry points should also be provided.

C.2.1.15 Ground hooks shall never be fused.

C.2.1.16 The ground hook connection to discharge points may be chain links, bare conductor or solid metal.

C.2.1.17 Provide fully visible, manual-grounding devices to render the capacitors safe while they are being worked on. Clearly mark discharge points and use caution to prevent transferring charges to other capacitors or reversing the charge on effected capacitors

C.2.2 High energy discharging

C.2.2.1 In equipment with stored energy in excess of >1000J, a discharge point with impedance capable of limiting the current to 500A or less must be provided. This discharge point must be identified with a yellow circular marker with a red slash, and/or must be labeled "HI Z PT" in large, readable letters.

C.2.2.2 The low-impedance points must be provided, whether or not the high Z discharge points are needed.

C.2.2.3 Calculations must be made to determine the discharge time of the circuit and the power requirements for the bleed circuit (see Example 1). The stored energy must be <5J before the low impedance ground hook is applied and the High Z ground hook removed (see Example 2). The calculations must be recorded in an Engineering note.

C.2.2.4 The resistors on the high Z ground hook must be rated for the voltage and energy of the circuit.

C.2.3 Workmanship, Finish, and Appearance

C.2.3.1 The external surface shall be uniform, symmetrical, and free of abrasions, scratches, blemishes, and surface defects.

C.2.3.2 Any defect that may capture an impurity or impair the dielectric integrity of the product shall be cause for rejection.

C.2.3.3 If a finish coating is applied to the ground hook rod or tube material, such as paint, the finished rod or tube material must meet all physical, electrical, and mechanical requirements.

C.3 Inspecting Ground Hooks

C.3.1 Inspection of ground hooks by QEW before each use

C.3.1.1 The ground hook must be visually inspected before each use. Refer to the Electrical Safety Manual for requirements. The Electrical Safety Manual can be found in PUB 3000, Chapter 8, Electrical Safety Program.

Note: Guidelines for the examination, cleaning, repairing, and in service testing of live-line tools are specified in the Institute of Electrical and Electronics Engineers IEEE Guide for Maintenance Methods on Energized Power Lines, IEEE Std 516-2009.

C.4 References

ASTM International, Standard Specification for Fiberglass-Reinforced Plastic (FRP) Rod and Tube Used in Live Line Tools (ASTM F711), West Conshohocken PA, ASTM International, 2013

ASTM International, Standard Specifications for Temporary Protective Grounds to be Used on De-energized Electric Power Lines and Equipment (ASTM F855-15), West Conshohocken PA, ASTM International, 2015

OSHA, Electrical Power Generation, Transmission, and Distribution (29 CFR 1910.269(j) Live-line Tools), Washington DC, United States Department of Labor, 2014

DOE, DOE Handbook, Electrical Safety (DOE-HDBK-1092-2013), Washington, DC, United States Department of Energy, 2013

LBNL, Electrical Safety Manual, Berkeley CA, Lawrence Berkeley National Laboratory, IEEE, Guide for Maintenance Methods on Energized Power Lines (Std 516-2009), Piscataway NJ, IEEE Standards Association, 2009

C.5 Examples

Example 1: Bleed resistor

Assume a pulser chassis with an enclosed 1uF capacitor at 1kV.

The stored energy is 0.5 * C * V^2 = 0.5J

Assume that the time to use a tool to open the cover and access the capacitor terminals inside the chassis is 30s.

5 RC time constants will discharge the capacitor to <1% of the charge voltage, so set this time to 30s.

5 * R * C = 30s

C = 1uF

R = 6Mohms (by calculation)

Check on the actual voltage on the capacitor at 30 seconds to verify the voltage and stored energy.

Vc = Vo * e^(-t/RC)

This bleed resistor will discharge the capacitor from 1kV to 6.7V in 30s once the charging power supply is off.

The stored energy in the capacitor at 6.7V is 0.022mJ.

Check the power dissipated in the bleed resistor during normal operation when the 1kV charging power supply is on to size the bleed resistor power rating.

P = V^2 / R = 1000^2 / 6Mohm = 0.17W

A 0.5W resistor which is rated for more than 1kV would be a conservative choice. It is also a best practice to use at least 2 parallel resistors for redundancy. If we use 2 parallel resistors in this case, each resistor would be 12Mohms.

Example 2: High Z ground hook resistor

Assume a power supply with a 20mF filter capacitor bank at 500V.

The stored energy is 0.5 * C * V^2 = 2.5kJ

When grounding out this capacitor bank, a soft ground point with a low Z ground hook or a high Z ground hook is required to initially discharge the capacitor bank and limit the discharge current to <500A.

500V / R = 500A

R = 1ohm

Although 1ohm would limit the current to 500A, any higher resistance value which would discharge the capacitor bank in a reasonable amount of time would also be acceptable. It is best practice to limit the current to much less than 500A to reduce the thermal stress on the resistor. For this example, 100ohms would be a conservative choice and would limit the current to 5A.

The 100ohm resistor must be capable of handling the peak power of 100V * 5A = 500W and the dissipation of 2.5kJ.

The capacitor bank voltage corresponding to 5J of stored energy is 22.4V.

0.5 * 20mF * V^2 = 5J

V = 22.4V (by calculation)

After application of the high Z ground hook, the time it takes to discharge the capacitor bank voltage from 500V to 22.4V is 6.2s.

Vc = Vo * e^(-t/RC)

22.4V = 500V * e^(-t/100ohms*20mF) t = 6.2s (by calculation)

The QEW must wait for more than 6.2s so that the stored energy is less than 5J before removing the high Z ground hook and applying the low Z ground hook.

C.6 Revision Log

Date, Revision # Brief Description of Revision / Changes Sec. #, Parag.

1/26/2016

Version 1 Initial version All

10/5/2016

Version 2

Changed usage to <1000 joules

Changed discharge to <5 J and < 50 V

Changed limit to >1000 J

C.1.1

C.1.2.3.c.i

C.2.2.1

DOCUMENT INFORMATION

Title: LBNL Ground Hook Safety Document number Revision number 2 Publication date: 10/5/2016 Effective date: 10/5/2016 Next review date: 10/1/2017 Last Edited By: pbong@lbl.gov EPG Section (home) Electrical Engineering References EPG Section (cross-reference) Functional Division Engineering