Remember the other day how we reported on Tesla Motors being proactive around overheating wall chargers for its Model S electric cars? Turns out there’s a recall issued by the National Highway Traffic Safety Administration (NHTSA) in effect that address this issue, and it is labeled by the government automotive safety entity as a possible fire concern.

image via Tesla Motors

The NHTSA, in the recall, said Tesla is addressing problems potentially impacting over 29,000 NEMA 14-50 (240 volt) Universal Mobile Connector (UMC) adapters. Here’s a description of the problem and a chronology of the events leading up to the recall from materials on the NHTSA website:

Description of Possible Defect
The NEMA 14-50 adapter plug designed for use with the UMC utilizes a series of blades connected to four pins to conduct electrical energy from a wall socket through to wires in the UMC cable to Tesla vehicles to allow recharging of the vehicle. The pins allow the NEMA 14-50 adapter to plug directly into the UMC through an interchangeable design in the UMC head unit. In the NEMA 14-50 adapter units, higher than normal electrical resistance connections to external energy sources may cause excessive heating of the adapter. In these situations, it is possible that electrical resistance heating in the NEMA 14-50 adapter or at the interface to the wall socket may lead to deformation of the adapter, cord or wall receptacle, and possible electrical arcing that could lead to fire.

Chronology of Principal Events
Since Model S deliveries began in earnest in late 2012, Tesla has received a very small percentage (2.7%) of returned UMCs that showed signs of internal damage only and that stopped vehicle charging. This was not a safety matter since damage was contained wholly within the UMC and resulted in cessation of power flow when the damage occurred – specifically, at the interface between the UMC and NEMA 14-50 adapter. In late 2013, Tesla became aware of several events that resulted in thermal damage external to the UMC. Based on this new information, the Company initiated an internal review of the UMC design, as well as the several external damage incidents, including a highly publicized event that took place in Irvine, California.  Initial analysis demonstrated that defective or improperly installed wall receptacles that the NEMA 14-50 adapter plugged into could cause problems including melted adapters and, in a worst case scenario, fire. While the number of incidents remains small, and Tesla’s review to date points to the building receptacle or wiring as the primary cause of failed NEMA 14-50 adapters, the Company has determined that a voluntary recall is appropriate as a precautionary measure.

In the steps Tesla already has taken to address this concern, an over the air software update first issued last month reduces the charging current by a quarter if unexpected power fluctuations happen. The company is also providing to impacted Model S owners a new charger with a thermal fuse that prevents current from flowing if overheating starts to happen.