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October 8, 2002

 

 

User Advisory

Advisory Title: Beamline Electrical Safety Guidelines
Advisory Number: 5/Rev. 4
Issue Date: January 1, 2002
Review Date: January 1, 2004
Current Info: This Advisory is valid for one year from Issue or Review Date(s), at which time it is reviewed for relevancy and accuracy. Copies of all current user advisories are available at the ALS User Services Office, (510) 486-7745.

Overview

Many devices used on beamlines at the ALS require high voltages (up to 10 kV). The guidelines described in this document are intended to help users avoid high-voltage exposure of humans or sensitive equipment. This document does not replace official Berkeley Lab and ALS procedures and policies concerning electrical safety. For more information on these policies, refer to Chapter 8 of the Laboratory's Health and Safety Manual, PUB-3000.

Vacuum Vessel Safety

Electrical hazards

When high-voltage components inside beamline vacuum vessels are energized, electric shock hazards can be created:

  • When a vacuum vessel is at atmospheric pressure, hands or electrically conductive tools entering open ports may contact high-voltage components, resulting in severe electric shock.
  • Any human-operated apparatus entering the vacuum vessel may contact high-voltage components, transmitting the high voltage to the operator as an electric shock.
  • A manipulator may contact high voltage inside the chamber, creating the possibility of severe electric shock to operators.
  • Low-voltage wiring within the vacuum vessel may contact high-voltage components. This may damage the low-voltage equipment, and it may also create a greater shock hazard by causing the low-voltage equipment to be energized to the high voltage.
  • Vacuum feedthrough connections whose return electrical paths run through the metallic vacuum vessel or its supports may become hazardous if a mechanical part is accidentally removed, breaking the return path and causing the vacuum vessel to go to the terminal source voltage and become an electric shock hazard.

Guidelines for Safe Operation

  • Design equipment with interlocks and other failsafes, rather than entrusting electrical safety to operator vigilance. Mechanical or electrical interlocks can be very simple. Members of the ALS Electrical Engineering Group have designed many types of interlocks and are a valuable resource for users.
  • Use vacuum interlocks or mechanical interlocks to turn off hazardous voltages when vacuum vessels are at atmospheric pressure or when ports are open.
  • Use mechanical barriers or electrical interlocks in designing human-operated equipment that will enter a vacuum vessel, to prevent contact with high-voltage components.
  • Ensure that low-voltage wiring inside a vacuum vessel is separated by a safe distance from any high-voltage components. Metal oxide varistors (MOV's), Transorbs", spark gaps, and similar devices can also be used to ground low-voltage wires if they exceed specified voltages.
  • Ground all manipulators, and restrict their motion, to avoid inadvertently charging them to high voltage.

Battery Boxes

Hazards

The Lab-designed "Battery Box" (a 3-kV supply consisting of ten 300-V batteries in series) avoids high-current hazards by placing a 300-kW resistor in series with each battery, thus limiting current to a safe level of 1 mA. Boxes from other labs, however, do not necessarily have this safety feature.

ALS Policy

When using high-voltage battery boxes that do not come with current-limiting resistors, users must add series resistors to limit the current to a safe level or provide an electrical or mechanical interlock. Berkeley Lab safety policy states that when exposed voltages exceed 50 V, maximum current must be below 5 mA to be considered safe.

MHV Coaxial Connectors

ALS Policy

MHV (maximum high voltage) coaxial connectors are not allowed at Berkeley Lab or the ALS because of their exposed high-voltage center pins; SHV (safe high voltage) connectors are recommended instead. MHV-to-SHV adapters, held in place with heavy-duty heat-shrink tubing, should be used to make the necessary conversions. The adapters (part number PE9335) can be ordered from Pasternack Enterprises [http://www.pasternack.com/main.htm; tel: (949) 261-1920].

High-Voltage Feedthrough Connectors

ALS Policy

There are many acceptable variations of high-voltage feedthrough connectors, as well as others for lower voltages. The goal is to use a feedthrough connector that allows the air-side mating cable connector to be inherently safe. The pins that are energized with hazardous voltages are recessed in the connector body and out of reach of a person's finger. Some examples of other feedthrough connectors that meet our safety standards, along with the part numbers and contact information for the companies that supply them, are listed below:

  • Ceramaseal (New York) Tel: 518-794-7800
    http://www.ceramaseal.com/
    Coaxial SHV (part number: 8880-02-CF)
    Multipin 12 kV (part number: 12867-05-CF)
    Coaxial 15 kV (part number: 8883-01-CF)

  • MDC Vacuum Products Corp. (California) Tel: 510-265-3500
    http://www.mdc-vacuum.com
    Coaxial SHV (part number: 634000)

  • ISI Insulator Seal (Florida) Tel: 941-751-2880
    http://www.isi-seal.com
    Coaxial SHV (part number: 9262000)
    Multipin 12 kV (part number: 9452000)
    Coaxial 20 kV (part number 9272006)
Approved by  Signed
Walter Barry, Electrical Engineering Group Leader 01/18/2002
Ben Feinberg, Division Deputy for Operations 03/15/2002