Which method of grounding cord-and-plug-connected equipment is NOT permitted according to Section 250.138?

The method considered not permitted according to Section 250.138 involves the use of a three- to two-prong attachment plug adapter. This type of device essentially allows a three-prong plug, which provides a ground connection, to fit into a two-prong receptacle that lacks a ground. By doing so, it compromises the ground path necessary for safety as it can lead to equipment being operated without a proper ground, thus increasing the risk of electric shock or fire.

Using a grounding adapter or an equipment bonding jumper are permitted methods because they ensure that the equipment remains grounded appropriately, maintaining safety standards. Direct grounding to the electrical panel is allowed as it provides a secure connection to the building's grounding system. The primary concern with the three- to two-prong adapter is that it can create a false sense of security regarding grounding, as it may not reliably connect the equipment to a proper ground point.