Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page Visit Our Archives Ask A Question How To Ask A Question Question of the Week NEWTON Teachers Our Expert Scientists Volunteer at NEWTON! Referencing NEWTON Frequently Asked Questions About Ask A Scientist About NEWTON Education At Argonne Double Insulated Appliances and Safety

Name: Azon R.
Status: student
Grade: 9-12
Country: Philippines
Date: Fall 2013

How does a double insulated appliance or power tool prevent electrical shock?

Thank you for your question. I am an electrical engineer who has designed double insulated power supplies.

The principle behind double insulation is redundancy. This is a common principle in any product where safety is a concern. A common example is that most commercial aircraft have at least two engines. This is done so that if one engine fails, they still have at least one more engine to fly the plane to an airport.

Where electrical safety is concerned, single insulation is an insulation system which is considered to be capable of protecting the user from electrical shock. However, if only single insulation is used and it fails, the appliance or tool may then cause an electrical shock. Just like the multiple engines on the airplane, double insulation is redundant. It uses two insulation systems (which may be similar or different from each other) so that if one fails the other can still protect the user.

Best regards, Bob Zwicker

Hi Azon,

Double insulation is simply a way to enure that an appliance's user does not receive a shock, even if there is a single failure of an appliance's insulation. The basic concept is that the appliance is insulated in the normal way to avoid shock, then a second, independent layer (such the insulating plastic housing on a hand drill) is added. In the event that some part of one insulation system fails For example, a failure of the insulation on internal wiring), the user is still protected by the second insulation layer or system.

One advantage of double insulation is that a double insulated appliance is that it is considered sufficiently safe that no so-called "safety ground" connection (that 3rd ground pin on an appliance's electric plug) is needed.

Regards, Bob Wilson.

Hi Azon,

Thanks for the question. A double insulated appliance or power tool has extra insulation (or shielding). This is so that the exterior or other user-accessible parts will not be at a lethal voltage in the event of a failure of the wiring. A double insulated appliance does not need a ground wire.

I hope this helps. Thanks Jeff Grell

Click here to return to the Engineering Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 223
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: November 2011
Weclome To Newton

Argonne National Laboratory