Cause of Device-Related Incident
Device factors; User errors

Clinical Specialty or Hospital Department
Clinical/Biomedical Engineering; Facilities Engineering

Device Factors
Design / labeling error; Device failure; Improper maintenance, testing, repair, or lack or failure of incoming inspection; Manufacturing error

Document Type
Hazard Reports

External Factors
*Not stated

Mechanism of Injury or Death
Electrical shock / electrocution; Failure to deliver therapy; Fire

Support System Failures
*Not stated

Tampering and/or Sabotage
*Not stated

User Errors
*Not stated; Abuse of device; Incorrect clinical use

UMDNS
Plugs, Explosion-Proof [15-848]; Plugs, Ground [15-849]; Plugs, Hospital Grade [15-850]; Plugs, Locking [15-851]

Electrical Plugs: A Compendium of Problems



Hazard [Health Devices Aug-Sep 1979;8(10-11):255-7]

The following is a summary of the information we have gathered from member hospitals to date, including what we have learned from several hazard investigations (see Health Devices, Vol. 2, p. 155; Vol. 3, p. 130; Vol. 4, p. 75; Vol. 8, p. 67; and Vol. 8, p. 147). These hazards and reported problems involved right-angle, locking, and straight parallel-blade plugs. While many of the occurrences were specific and perhaps isolated incidents, they can be grouped into the following general problem areas:

  • Terminal screw loosening
  • Line cord shearing at strain relief
  • Misuse or abuse
  • Production defects and quality control problems
  • Inappropriate application

Terminal Screw Loosening

The most frequently reported problem is the loosening of terminal screws in plugs, resulting in loss of power and/or loss of ground continuity or overheating of the plug. This problem can usually be traced to tinned wires or to inadequate tightening. Tightening a terminal screw on a tinned wire exerts forces that cause "cold flow" of the solder. The wire-solder combinations relax and change shape, and the terminal screws loosen. This type of cold flow may also occur where solid, rather than stranded wire is used. Inspect incoming equipment for tinned leads on power plugs. If you find any, cut off the tinned portions and rewire the plug. Advise the manufacturer and ECRI so that we can help eliminate the practice of tinning wires.

Inadequate tightening occurs because some maintenance personnel do not know how tight the screws should be. Some manufacturers have recommendations on their installation instructions; we think that 10-14 inch-pounds of torque is adequate. We do not suggest using a torque screwdriver for each installation, but each installer should practice with one to get the "feel" of the required tightening torque.

Loose terminal screws may sometimes be detected by the "rattle test" (shaking the plug and listening for rattles indicative of loose screws). However, this test may not detect screws that are only slightly loose, and plugs should be inspected annually (coordinated with inspection of the equipment). We do not advocate routine disassembly, but disassembly may be necessary if the rattle test or ground continuity checks indicate problems.

Line Cord Shearing at Strain Relief

To provide strain relief for a wide variety of cord diameters, plug manufacturers use two types of adjustable cord grips: crimp and chuck. Crimp types secure the wires by bending or "detouring" them inside the grip. This helps prevent cord pull-out, but more strain is placed on the wires when the cord is flexed. We have seen stranded conductors in which some or all of the strands have broken as a result of overtightened crimp-type grips and crimps with unnecessarily sharp edges. Line cord shearing may also result from failure to take out the removable inserts when installing a large-diameter cord. The plug manufacturer should indicate which cord diameters require the inserts. If this information is not provided, call the manufacturer.

The chuck type of cord grip, though more universal, also has cord size limitations. We have received no reports of line cord shearing with this type of grip, but line cord pull-out may occur if the grip is not adequately tightened. At this time, we do not have enough information to determine which type of cord grip is better.

Misuse or Abuse of Plugs

We have received several reports of plug damage that apparently resulted from using the plugs in an improper environment. Our most recent example of misuse is the sterilization of plugs in 88% Freon, 12% EtO (Health Devices, Vol. 8, p.147). Polycarbonate components of plugs are not compatible with the Freon used in some sterilizers, and repeated sterilization will cause failure of these components when they are stressed. Before cleaning plugs, check with the manufacturer to determine which cleaning agents degrade plug materials. When cleaning, take care to prevent fluids from infiltrating the plug.

Hospital grade plugs, though much more rugged than older types, are not indestructible. Member hospitals have reported cracked bodies and bent blades that appear to be the result of abuse. Never pull on the line cord to remove a plug from a receptacle. Do not use right-angle plugs on equipment that is frequently plugged and unplugged. The wire in right-angle plugs parallels the plane of the wall to save space, and improper removal places even more strain on both plug and receptacle than occurs with straight plugs. If you have equipment that may be unplugged in an emergency situation (e.g., crash carts) consider using a Phoenix Safety Star (Health Devices, Vol. 7, p.227). This device attaches to the plug body and acts as a lever, so that stresses applied to the cord are not borne solely by the plug receptacle combination.

We also received reports of improper wiring of plugs (reversal of hot and neutral leads). All strands of the conductor should be clamped by the wire clamp or terminal screw during installation. We've seen cases of excessive heating caused by only partially clamped wires. Be careful to install plugs properly.

Production Defects and Quality Control Problems

Because plugs are mass produced, production-related defects can affect many devices. Reported production defects include cracked parts, interchanged brass and silver blades, and stripped terminal screws. Until recently when some manufacturers began date-coding, a defective plug could not be linked to a production lot. We have received many reports of isolated defects, involving various types of plugs, which could not be traced. Recalls of plugs are difficult even when a problem is traceable.

Inappropriate application

Nonhospital grade molded-on plugs (two or three prong) should not be used in patient-care areas. Live-front plugs, in which current-carrying parts other than the blades or pins are exposed are no longer manufactured or listed by Underwriters Laboratories (UL). They should not be used anywhere. The National Electric Code (NEC) requires use of dead-front plugs, which are wired from the rear, away from the contact blades.

The use of two-prong and nonhospital grade molded-on plugs in non-patient-care areas presents a dilemma. Many double-insulated devices are not designed to be grounded, and replacing plugs on this equipment is neither cost effective nor necessary. While the safety of the hospital support staff is important, electrical safety requirements and use environments are quite different for patient-care and other areas. However, equipment that is frequently plugged and unplugged should have hospital grade plugs regardless of where it is used.

We have also discovered that hospitals use locking plugs in areas where they are unnecessary. The controversy surrounding the applications of locking plugs and receptacles should be resolved with the adoption of  National Fire Protection Association (NFPA) NFPA 56A-1978. Section 3-4.2.1 will allow wiring devices "listed for the use" except in hazardous areas of flammable anesthetizing locations. (We understand "listed for the use" to mean hospital grade parallel-blade plugs and receptacles.) This agrees with the 1978 NEC, sections 517-103b and 517-102e, which requires locking plugs and receptacles only in hazardous areas. Hospitals that no longer permit flammable gases in their operating room suites do not need locking devices at all. Check with your state and local officials to determine what rules govern your hospital.

Other Comments

The quality of plugs varies widely from hospital grade to commercial and residential grades. UL and the National Electrical Manufacturers Association (NEMA) have regulations regarding the design and performance of plugs. As with receptacles (Health Devices, Vol. 8, p.3), UL has special performance requirements for listed hospital grade devices. NEMA requirements involve design and compatibility specifications.

Through NEMA, wiring device manufacturers have established standard configurations for electrical connectors and have made these devices incompatible, where necessary, to prevent misapplications. A sound rationale exists for incompatibility. For example, a 20-amp plug on a 20-amp-rated device should not be replaced by a 15-amp plug in order to be compatible with a certain receptacle. The circuit should be of suitable capacity and equipped with the appropriate receptacle. Defeating this system could cause nuisance tripping of branch circuit breakers or overheating of the power distribution system.

Plugs of different sizes and ratings may be found on various equipment, depending on the electrical requirements and use environment. While most hospital personnel are familiar with 15-amp, 125-volt locking and nonlocking plugs, they may not be familiar with some other plugs, such as those found on treadmills, food carts, and x-ray units.

When selecting a plug, hospitals should consider the following:

  • Does it have adequate wiring instructions?
  • Does it have strip gauges?
  • Is it easy to install?
  • Is strain relief adequate?
  • Is it easy to grasp?
  • How well does the plug resist fluid infiltration?

Clear plugs, sometimes advertised as "see-through," are expensive, and their transparency helps only in detecting gross wiring errors or damage. In some, the terminations cannot be seen, even when the plug is new, and these plugs often collect dirt, which obscures the terminations anyway.

Plugs are a vital part of the electrical circuit, and defects or misuse can seriously compromise the safety of patients and hospital personnel, as well as harm the equipment in the circuit. Hospitals must ensure that plugs are selected and installed carefully and inspected regularly.

UMDNS Terms

  • Plugs, Explosion-Proof [15-848]
  • Plugs, Ground [15-849]
  • Plugs, Hospital Grade [15-850]
  • Plugs, Locking [15-851]

Cause of Device-Related Incident

Device factors: Design/labeling error; Device failure; Improper maintenance, testing, repair, or lack or failure of incoming inspection; Manufacturing error

User errors: Abuse of device; Incorrect clinical use

Mechanisms of Injury or Death

Electrical shock/electrocution; Failure to deliver therapy; Fire


[Home]    [About]    [Help]    [Site Map]
Copyright © 2017 ECRI
All rights reserved
www.ecri.org