ES&H Manual



6240 Appendix T1

Electronic Equipment – Best Practices



1.0             Purpose


Often Jefferson Lab has need of unusual electronic equipment which does not meet the standards and listing requirements set forth for industrial applications. In order to ensure that such equipment is safe in accordance with Jefferson Lab’s acceptable safety standards, best design and construction practices are provided within this appendix. These are to be reviewed and incorporated as applicable into department procedures.


2.0             Scope


This appendix provides best work practices for safely constructing, modifying, and maintaining electronic equipment. The extent to which these practices are applied is determined by the person responsible for the design, construction, and commissioning of the equipment.


Items that have not been altered, and are certified by a Nationally Recognized Testing Laboratories (NRTL) (e.g.: those that are UL listed), meet the requirements of this document. 


Modification to NRTL listed equipment invalidates the listing. If certification needs to be restored, a recertification inspection is required.


3.0             Responsibilities

NOTE:           Management authority may be delegated to a task qualified Jefferson Lab employee at the discretion of the responsible manager.


3.1              Supervisor/Subcontracting Officer’s Technical Representative (SOTR)/Sponsor

·         Ensure that only qualified workers design or modify electronic equipment and the work is accomplished as specified in ES&H Manual Chapter 6230 Electronic Equipment Safe Work Program.

·         Ensure appropriate level of review, construction, and modification is performed as needed.

·         Ensure inspection of final product.


3.2              Designers

·         Obtain appropriate authorization to design or modify equipment.

·         Comply with appropriate standards or guidance.

·         Reduce hazards by incorporating engineering safeguards, and reducing mode or class levels.

·         Create and maintain proper documentation of design in accordance with department procedures. For example:

o   Specifications

o   Design analysis and calculations

o   Drawings

o   Schematics

o   Software

o   Pressure system procedures

o   Wiring diagrams

o   Safety Analysis

·         Schedule, perform, and document appropriate design reviews.


3.3              Technicians


4.0             Expectations


4.1              Minimum Expectations

Any work activity at Jefferson Lab follows the same minimum over-reaching safety procedures. These include:


·         Performed tasks in accordance with principles recognized by the industry as good practice, consistent with consumer safety. 


·         Provide for an equivalent level of protection if recognized or harmonized standards cannot be strictly followed.


·         Meeting the intent of applicable electrical safety standards.


4.2              General Electronic Safety

It is Jefferson Lab’s preference to use NRTL listed electronic equipment for all equipment needs. When this is not possible, the following considerations comprise a non-exhaustive guide:


·         Voltage Verification Units (VVU)

Incorporate wherever possible as a means to mitigate Mode 2 to Mode 1 on Class 2 or above equipment. VVUs greatly reduce the exposure to electrical energy during Lock, Tag, and Try; and assist with the standardization of the procedures for de-energizing electrical equipment.

o   The VVU operation must meet the requirements of the VVU equivalency.


·         Appropriate Connections to Electrical Distribution Equipment

This includes proper grounding, respect of polarity, insulation, and the reduction of Mode and Class, wherever possible. 


Implement as necessary commensurate with level of complexity and hazards.


Reduce whenever possible.


Use to indicate the status of the circuit.


Design and construct appropriate personnel protection to prevent unintended access to energized circuits.


Use as suitable for any wire or cable penetrating an enclosure. Grommets or similar devices are unacceptable for use as strain relief.


Consider area lighting required for larger pieces of equipment.

o   Incorporated lighting into the design, or

o   Provide for a means for temporary illumination.


Per National Electrical Code (NEC), or consistent with existing wiring (e.g. modifications to imported products) (avoid confusion whenever possible).


Consider the design, construction, and proximity of other electrical components. If unavoidable, use an appropriate method of containment.


4.3              Disconnects

·         Provide for overload protection and maximum fault current interruption.

·         Incorporate local “off” controls particularly on remote-controlled equipment.

·         Incorporate power switches on chassis so power can be removed without unplugging the device, wherever feasible.

·         Clearly label loads and associated controls. When 50VAC or above is used as a control voltage, the control voltage must be clearly marked.

·         Design for Lock, Tag, and Try.


4.4              Overload protection

·         Utilizes appropriate over-current-protection on inputs and outputs.

·         Employs only approved fuse holders or circuit breakers.

·         Is properly rated and devices are labeled.

·         Adequate accessibility for maintenance.


4.5              Remote Control Devices

·         If failure could create a hazardous condition then a “fail-safe” is designed into the initial configuration.

·         In the event of a fault in a main system containing breakers, fuses, interlocks or momentary on-off switches, the control circuit seals or latches to prevent the release of energy or re-energization and aid in fault identification and manual reset.

·         Control power is isolated from higher-power circuits by transformers, contactors, or other means.

·         Control power is limited to no more than 120 volts, AC or DC except on previously installed equipment.[1]


The standard Jefferson Lab control voltage for the accelerator is +24 VDC.


·         Hazardous additive voltages are not produced between control circuits derived from line power having different phase or polarity.

·         Control-circuit currents do not exceed 5-amperes, and are limited by fast-acting fusing, circuit breakers, or power-supply rating.


4.6              Chassis/Enclosure Construction

·         Chassis components are specified in accordance with their intended environment. They are designed to be sturdy enough to withstand an appropriate qualification test.  Chassis materials are non-combustible, and/or non-flammable. They do not result in toxic or poisonous fumes. Internal power supplies are secured, such that they will survive the appropriate qualification testing.

·         Direct contact of Class 2 and Class 3 hazards are prevented by the use of covers and/or enclosures, so as to isolate the source.

o   Class 2 enclosures may consist of voltage rated plastic barriers or covers.

o   Class 3 enclosures and chassis systems must include in their source isolation:

§  Screw-on panels, possessing a minimum of four screws or bolts, or

§  Locked and/or electrically interlocked doors or panels



(See ES&H Manual Chapter 6230 Appendix T1 Determining Equipment Class and Work Modes)


·         The frame or chassis of an enclosure is connected to a good electrical ground with a conductor capable of handling any potential fault current. An enclosure may be a room, a barricaded area, or an equipment cabinet.

·         Easy open doors, panels, etc., that permit access to exposed hazardous circuits, are interlocked so that the act of opening de-energizes and automatically discharges the stored energy. 

·         Enclosures physically prevent contact with live circuits. Enclosures can be constructed of conductive or nonconductive material. If conductive, the material must be electrically interconnected and connected to a good electrical ground. Connections must be rated to carry all potential fault currents.

·         Enclosures are designed to contain flying debris caused by explosive component failure.

·         Ventilation is sufficient to prevent overheating of equipment and to purge noxious toxic fumes produced by an electrical fault.

·         Ventilation openings are designed to minimize the risk of accidental contact through the openings, e.g., use of small mesh grille.

·         Enclosures large enough to be occupied by personnel allow for exterior observation of equipment and personnel working inside the enclosure.

·         Appropriate signage and labeling is applied to identify hazards, functions, and operations.

·         A keyed switch can be used in each interlock chain involving personnel hazards, or major equipment hazards to assure power removal before anyone enters the enclosure. This same key is also used to gain access to the controlled equipment. This does not replace the requirement for the Lock, Tag, and Try Procedure.


Although not strictly required at the time of this writing, the use of NEMA (Association of Electrical and Medical Imaging Manufacturers; formerly National Electrical Manufactures Association) approved enclosures are strongly encouraged for Class 3 source isolation systems.


See: www.NEMA.org or http://www.nema.org/prod/be/enclosures/upload/NEMA_Enclosure_Types.pdf


4.7              Class 2 & 3 Equipment

·         Use self-contained power supplies to protect personnel from line-level voltage when equipment is energized.

·         Any hazardous voltage present in electronic equipment is protected, such that individuals are not exposed to live conductors. Conductors carrying hazardous voltages are not accessible, without the use of tools and proper Personal Protective Equipment (PPE). 

·         Heat-generating components, such as resistors, are mounted so that heat is safely dissipated and does not affect adjacent components.

·         All conductors, switches, resistors, etc., are operated within their design tolerances. Pulsed equipment does not exceed the average, the Root Mean Square (RMS), or the peak rating of components. The equipment is de-rated as necessary for the environment and the application of the components. Exceptions are clearly described in relevant documentation.

·         Equipment having an Emergency-OFF switch is clearly identified. The switch is within easy reach and resetting it is not automated.

·         Distinctively color and/or label components that are commonly non-hazardous if they could become hazardous when used in a unique application (e.g.: a copper pipe carrying high voltage or high current). 


5.0             References

·         NFPA 70E - Standards for Electrical Safety in the Workplace

·         NFPA 70 - National Electric Code

·         OSHA Regulation 29 CFR 1910 and 29 CFR 1926

·         DOE Electrical Safety Handbook

·         MIL-HDBK-781A (01-April-1996) Department of Defense Handbook for Reliability Test Methods, Plans, and Environments for Engineering, Development, Qualification, and Production


6.0             Revision Summary


Revision 1.3 – 11/14/18 – Periodic Review; Deleted reference to NFPA-70E (2004 edition) per CATS# STR-2017-12-06-04

Revision 1.2 – 11/12/15 – Periodic Review; no substantive changes

Revision 1.1 – 05/20/12 – Periodic Review; updated format with no substantive changes

Revision 1.0 – 05/20/09 – Update to reflect current laboratory operations











ES&H Division

John Musson





This document is controlled as an on-line file.  It may be printed but the print copy is not a controlled document.  It is the user’s responsibility to ensure that the document is the same revision as the current on line file.  This copy was printed on 1/28/2019.

[1]Previously installed equipment (installed prior to March 2009) which has control power at more than 120 volts is “excluded” from this restriction; however, the equipment must be clearly marked with the control power voltage.