Is it safe to use an adapter to plug a three-prong cord into two prong outlet?
While these adapters are sometimes used to make this type of connection, there is no longer a safety ground connection to the electrical system. Ideally appliances and equipment with three-prong cords should only be plugged into grounded outlets. In some cases, where grounded metal boxes were used for the original installation, these boxes are grounded and a three-prong receptacle when properly installed provides the necessary ground connection. The electrician can determine this with a simple meter test. In cases where the box is not grounded and only a two-wire circuit has been installed, the correct solution would be to install a new cable back to the main electrical panel.