A recent Consumer Product Safety Commission alert is urging people to not use male-to-male power chords. In spite of being illegal, the product is available online with the supposed use to feed electricity from a generator into a building’s main power system. The consequences of doing so, however, could be dire. Many hardware stores have also warned consumers over the years that male-to-male “adapters” for things like holiday lights are unsafe, but sometimes these admonitions go unheeded, resulting in preventable accidents.

Once a cord is plugged into a power source, the other end of it becomes “live,” and with a male-to-male connector, the metal tines of the plug become electrified and cause a serious shock risk. The ends will also electrify any conductive object they come in contact with, including household objects made from metal. That means that if you come in contact with a live cord, you could get 120v worth of a shock–enough to kill you under the right circumstances. If the live end of the cord isn’t attached to a breaker or any other safety shutoff, the electricity won’t stop once the circuit is overloaded, so the potential for electrocution under these circumstances is greater than under normal conditions.

Unfortunately, electric shock is only one of many ways that a double-male extension cord can harm you. If you’re using it to try and electrify your home’s system with a generator, it will be running for at least some of the way in the opposite of its intended direction. That means that safety features like circuit breakers will be bypassed and can cause a fire, resulting in catastrophic damage to the building it’s plugged into, not to mention serious injury or death to anyone inside. A fire can also start at the generator if the power is restored while it’s connected to the house, resulting in a generator fire with lots of fuel to burn.

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Bypassing the standard safety systems isn’t only dangerous to those on the property using the generator. Utility workers who are trying to restore power to an area experiencing an outage are also in danger, as the power lines they’re working on can become unexpectedly electrified. Even if you turn off the main breaker to your home while the generator is in use, without a proper safety system in place the breaker can be accidentally switched back on.

Another hidden danger of using a short, double-ended adapter like the ones being sold online to power generators is one you might not have considered: The Consumer Product Safety Commission warns that gas-powered generators running so close to your home might increase your risk of carbon monoxide poisoning due to the fumes from the generator.

The best practice, obviously, is to leave home wiring to the pros. While an adapter might seem like the perfect fix for a wrong-direction string of Christmas lights, the possibility of the male end being exposed while still connected to the outlet makes this plan more danger than it’s worth. Keep yourself and your neighbors safe by never using these dangerous adapters unless you’re a certified electrician.