Technical bulletins
November 2013

Block Heaters: the Risks of Comfort!

By Daniel Roy P. Eng., CFEI, CVFI, Martin Morissette Eng., CFEI, CVFI,

Figure 1: Block heater connection

In Canada, many vehicle owners use block heaters to assist with their winter starting and to kick-start the heating of the passenger compartment (Figure 1). Unfortunately most people do not realize that this accessory could represent a real danger if not installed or used properly.

First, we will provide some basic background information. A block heater is in reality nothing more than a simple electric heating element with a power cord, which is designed to connect to a regular 15-amp outlet (Figure 2). This heating element is installed in the engine block cooling system circuit.

Of all the parts comprising the block heater, the power cord is by far the most vulnerable as it is often sub­jected to adverse conditions that may affect its life expectancy. These conditions are related to issues arising from improper installation or securing, exposure to elements and possible misuse by the user.

The following provides some basic expla­n­ations and possible ensuing consequences for the most common situations involving block heater power cords.

Figure 2: Block heater parts: heating element and power cord

1- Improper installation or securing

A block heater is generally installed by a mechanic (dealership), but a vehicle owner could perform this task on their own. The installer (professional or not) may not always take the time to read and/or follow all the installation instructions correctly, which could result in the power cord being routed impro­perly within the engine compart­ment and ultimately installed too closely to a hot surface. The power cord could also be inadequately secured, which could cause it to rub against sharp metallic parts and lead to mechanical damage. These two situations could cause local and premature wear of the power cord, which can lead to a catastrophic failure of the cord.

2- Exposure to the elements

The power cord end plug is often exposed to the elements since most of the time it is installed through the front grill of the vehicle in order to be easily accessible without having to open the hood. The end plug is normally provided with a cover, but if the cover is missing, the plug's blades are then exposed to water, dirt, salt, as well as other pollutants that could contribute to the oxidization of the metal. Oxidized metal can result in poor electrical conductivity and is a common cause of high resistance connections. In such a case, localised overheating may develop over time at the connection point, which, if left undetected, may lead to a catastrophic failure of the cord near the end plug of the cord.

3- Possible misuse

In a typical scenario where a car is parked in the driveway of a home, the block heater's power cord is connected to an extension cord which is plugged into an exterior receptacle. Unfortunately, many drivers have at least once forgotten that their block heater was still plugged in as they drove off. As a result, the power cord and the connection points can incur significant mechanical damage. At first glance, such an incident might appear insignificant, but it could stretch or even break the power cord or cause damage to the connections (plugs). Extension cords and power cords are made of flexible stranded conductors.

Figure 3: Original installation
Figure 4: Damaged power cord or connection

Although no damage is visible after the event, such tension on the cord can sever some of the strands of a current carrying conductor, thereby reducing its capability of carrying its rated current. In simple terms, a conductor whose cross-section has been reduced by broken strands will have a tendency to overheat if a sufficient amount of current is passing through it.

As shown by the simplified circuits in Figures 3 and 4, if a power cord or a connection is damaged, its impedance (resistance) will increase. In this hypothetical scenario, damage to the power cord is assumed to result in an increase of resistance from 0.01 to 1 ohm. The current as well as the total power of the circuit will decrease, but the power dissipated by the damaged power cord or connection will increase from 0.36 Watts to 32.5 Watts.

This simple example shows that a damaged cord may end up with a situation where 90 times more power (0.36 W to 32.5 W) is dissipated in a specific section of the circuit (near the damaged portion of the power cord). Localized overheating of the damaged power cord could potentially result in failure and cause the ignition of combustible materials in the area.

The above example also shows that an overcurrent device, typically a 15-amp breaker, may prove ineffective to prevent a failure as the total current of the damaged circuit is only 5.7 amp.

In many cases, due to the proximity of the subject vehicle to a building, a fire resulting from a block heater power cord may not only affect the vehicle but could also cause severe damage to surrounding properties.

Examinations of such fires require multiple fields of forensic engineering expertise. The forensic experts of CEP Forensic Engineering Inc., having performed many examinations of this nature, are well positioned to provide a multi-disciplinary investigation to help you Target the Cause of the fire.