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Monday, 20 June 2011 06:19 Written by redfordelectric
Issues with knob and tube wiring
Since January 2003 the Electrical Safety Authority has received an increasing number of questions about the safety of knob and tube wiring. In particular, purchasers or owners of older homes are finding that many insurers will not provide or renew coverage on such properties. In some cases, the insurance companies are requiring a total replacement of this wiring prior to providing insurance coverage.
Knob and tube wiring, more recently referred to as open wiring, was a wiring method used in the early 1900s to 1940s in the residential sector. Over the years wiring installation practices have changed in the residential sector and knob and tube wiring is no longer installed; however, parts continue to be available for maintenance purposes.
Diagram F1- Typical knob and tube installation
Existing knob and tube conductors concealed in walls, floor spaces, etc; supplying general lighting and receptacle circuits are permitted to remain in place if:
If your home has knob and tube wiring, we recommend that you follow these guidelines:
• Have a Robert Redford Electrical Services check the “knob and tube” conductors in your existing installations for sign of deterioration and damage; or request a general inspection from ESA. The General Inspection report will identify visible electrical safety concerns in your electrical wiring.
Homes with knob and tube wiring may not have the electrical capacity to meet today’s needs. As a result, homeowners have modified their electrical system with what the Electrical Safety Authority classifies as unsafe practices:
Homeowners who are planning to modify their knob and tube wiring, or any other electrical wiring, should have the work performed by a licensed electrical contractor or electrician
Receptacles in existing knob and tube installations
Where grounding type receptacles (three pin) are installed in existing knob and tube installations to replace the ungrounded type (two pin) receptacle, special caution must be exercised.
Diagram F2-Two and three pin receptacle configuration
Two Pin (ungrounded) Three Pin (Grounded)
Rule 26-700(7) requires the installation of a bond conductor to bond the receptacle to ground. This is permitted to be an external bonding conductor that is connected to either the system ground conductor or a metallic cold water pipe that is bonded to ground. This method may be difficult to accomplish.
As an alternative to bonding, Rule 26-700(8) of the Code also states "grounding type receptacles without a bonding conductor shall be permitted to be installed provided each receptacle is protected by a ground fault circuit interrupter of the Class A type that is an integral part of this receptacle; or supplied from a receptacle containing a ground fault circuit interrupter of the Class A type; or supplied from a circuit protected by a ground fault circuit interrupter of the Class A type (a GFCI breaker in the panel, or either a GFCI receptacle or a GFCI dead front mounted in an outlet box next to the panel). Where this option is used, no bonding conductor is permitted between outlets unless that bond conductor is in turn connected to ground. GFCI protection of the receptacles does not provide a ground reference to the U-ground slot of the receptacles. Some appliances require a bond be connected to the U-ground slot in order to function properly. For example, surge protective devices for computer or entertainment equipment will not function without a ground reference.
As new electrical equipment is introduced into the dwelling unit there might be a need for additional outlets to be installed. Extension cords are not to be used as a substitute for permanent wiring. The following shall be followed when installing new receptacles:
Benefits of new wiring
While knob and tube conductors in good condition and has not been inappropriately altered will not present undue hazards it is worth noting that modern electrical installations contain safety benefits not found in older electrical systems.
Homeowners who are planning to modify their knob and tube wiring, or any other electrical wiring, should have the work performed by a licensed electrical contractor or electrician. Knob & Tube wiring is unsafe.
Facts: Knob & Tube wiring is safe, provided it is properly maintained by competent licensed people as outlined above.