Generators & Automatic Transfer Switches

Transfer switches are a critical component of emergency power systems. Typically, any generator set that is used as a standby power source will be connected to a transfer switch in the event of a power outage. Generators and transfer switches are used together to assure the continuity of vital electrical power. Transfer switches and generators can help avoid loss of revenue for a variety of businesses such as data centers, restaurants, or climate controlled storage facilities where power is necessary to prevent inventory loss. Without a properly installed emergency generator power system, even a few minutes of power loss can be devastating to these types of businesses. On top of that, generators and transfer switches can help prevent the loss of life for hospital, police and security, or airport applications. In hospitals, power failures can be fatal to patients who depend on intensive care units, operating rooms, and other life-sustaining equipment that relies on electricity. Without a reliable transfer switch, an emergency generator will not be able to perform its primary function of providing power during a utility power failure.

The purpose of a transfer switch is to switch the power source during the event of a loss of power from the grid, to power from a secondary source such as an emergency generator. Emergency power can be supplied by multiple utility services, or by an on-site generator. However, for maximum control and reliability, emergency power is most commonly provided by wither a natural gas, liquid propane, or diesel generator set.

There are several types of transfer switches, such as manual switches, open transition (delayed) automatic transfer switches, closed transition automatic transfer switches, and bypass isolation transfer switches.

Manual transfer switches are typically used for non-emergency type loads, as they require personnel to manually operate the switch to change power sources. Some typical applications for manual switches would be residential use, certain control centers, or non-critical healthcare facilities. They are also popular with portable generator sets that can be rolled up to a building and then have the power transferred over. Manual transfer switches often must be operated under no load conditions, meaning that the load must be disconnected either by opening breakers or another load interruption before the switch is operated to move to generator power.

Automatic transfer switches, unlike manual switches, sense the loss in power, and automatically switch the power over to the secondary power source, often an emergency generator. There are 7 major functions for an automatic transfer switch; carry the current continuously, detect power failures, initiate the alternate source such as a generator, transfer the load over to the generator, sense the normal power source restoration, re-transfer the load from the generator to the normal source, and also be able to withstand and close on fault currents. As the previous sentence suggests, the transfer of power does not happen instantaneously. Open transition automatic transfer switches change power over to the secondary power source (generator), but first the generator receives a signal from the ATS that it needs to start its engine, and then once it reaches the proper voltage and frequency, power is switched over to the generator. Once the ATS then senses the power is back from the primary power source, then the switch is operated again to bring back that primary power source and sends a signal to the generator to shut down. This delay from primary power source to the generator typically lasts under 10 seconds, however the delay from the emergency generator back to the primary source of power is often set at 30 minutes. This is to allow the primary source to stabilize before transferring back the load from the emergency generator to the primary source (typically a utility). The ATS is always powered from the source to which the load is being transferred. This means that an automatic transfer switch always has current running through it.

Closed transition automatic transfer switches actually parallel both the primary and secondary power sources for a period of time no greater than 100 milliseconds. This means that the electrical load is not disconnected from power during a transfer. Closed transition transfer switches are often used in anticipation of a power outage, which makes them great for testing an emergency generator system. However, closed transition requires that both the emergency generator system and the primary power source are in sync with their voltage, frequency, and phase angle. These types of switches are used when there can be a zero interruption of power. In order to ensure that the emergency generator load is in sync for this sort of transfer, the engine on the generator will be required to be controlled by an isochronous governor. The voltage differential must be within 5%, the frequency differential must be within .2 Hz, and the phase angle difference must be within 5 degrees for proper function. Typical applications for this sort of transfer switch would be data centers, certain motor and transformer loads, or any type of load where a power interruption would be objectionable.

Automatic transfer switches with bypass isolation means that the unit has both an automatic, and manual transfer switch. The bypass switch is typically the manual switch. These types of units are the most expensive of the switches mentioned in this article. They are great for testing and maintenance purposes, as you can manually transfer a load over, and then perform maintenance on the automatic transfer switch without breaking the electrical load. It also allows manual transfer of the load if power fails when the ATS is disconnected from the load. There are three main types of bypass isolation switches; one momentarily interrupts the electrical load during bypass, one bypasses without any load interruption, and the final type is built of 2 automatic transfer switches that are in parallel together. Keep in mind that these types of transfer switches take up much more space than a transfer switch without bypass isolation capability, and it is typically specified that there must be space for both front and rear access for maintenance.

There are several factors to consider when choosing which type of transfer switch to use in a given system. Most importantly, a given ATS must be able to perform the tasks listed below.

1) Close against high inrush currents

As mentioned above, the secondary source of power (generator) may not always be in sync with the primary source. If the load that is required has a high initial current draw, such as a motor load, the ATS must be able to close on up to 13-20 times the normal operating current (amperes) of the motor.

2) Interrupt current.

When the contacts within a transfer switch change from the primary power source to the generator, or vice versa, there may be a small arc that is drawn from the source the contacts are leaving. This arc must be extinguished before the contacts switch over to the generator load, or there is a high possibility of a short circuit. The transfer switch must be able to interrupt the arc before the switch is made. This is typically done by having a sufficiently wide arc gap between the contacts which switch the power. Another way to interrupt the arc is by having what are called arc splitters, within the transfer switch. These splitters are magnetically drawn to the arc when it is formed, and help extinguish the arc upon contact.

3) Carry the full rated current on a continuous basis.

Transfer switches are different from other equipment within an electrical system, as they are continuously carrying an electrical load, whether that power source is from an emergency generator, or from a utility. Transfer switches typically facilitate a continuous power flow for a minimum life of 20 to 40 years. This is keeping in mind all of the switching that will come about as a result of regular maintenance testing, and in the event where there is an actual power outage. The transfer switch unit has to perform its job. Reliability is critical for both the transfer switch and the emergency generator set, as they a meaningless expense if they don’t perform during a power outage. To avoid equipment failure, be sure to look at what loads the transfer switch will be expected to handle to avoid overheating. This goes back to being able to handle a sudden inrush of current. When choosing a transfer switch, or a generator set for that matter, it is also important to keep in mind if any loads will be added to the electrical system in the future, especially if they will be critical loads that the generator set will be expected to supply power to in the event of an emergency.

4) Withstand and close against fault currents.

A fault current is an abnormal electrical current that passes through an electrical system. Transfer switches must be able to handle instances of high heat and magnetic stress that can be sent its way. Overcurrent exists when the current going through a system exceeds the rating of the conductors or the equipment in that system. Causes of overcurrent could be a short circuit, or ground fault. Transfer switches have to be able to withstand and close on the available fault current at its location until an overcurrent protection device such, as a circuit breaker or fuse, clears the fault. The person or entity that designed the electrical system should be able to determine what the necessary rating of the transfer switch should be, based on calculating the symmetrical fault current magnitude (X/R ratio), the root mean square (RMS) of the symmetrical fault current, the voltage at the transfer switch location, and the type of fault clearing device that will be present. If these calculations are not done correctly, improper sizing and rating of a transfer switch can occur.

Enclosures on a transfer switch are just as critical as the enclosures of a generator set. Without the proper enclosure, a transfer switch can easily be damaged and render the emergency power system useless. Not unlike generator sets, there are several different enclosure types for transfer switches, depending on the application. Enclosures are rated by the National Electrical Manufacturers Association (NEMA). While there are a variety of enclosure types for just about every environment on the planet, some of the most common are as follows.

Type 1 Enclosures are built for indoor use to, and prevent most dust and debris from entering and affecting the mechanical parts. This enclosure also prevents operators from being directly exposed to the mechanical and electrical parts unless the enclosure is open. Keep in mind this is built for indoor, not outdoor use. If left outside, it will not be sufficiently protected from any falling rain, sleet, snow, or blowing dust. 

Type 3R Enclosures are rated for both indoor and outdoor use. They are able to withstand falling rain, sleet, snow, and most dust particles. This enclosure also prevents operators from being directly exposed to the mechanical and electrical parts unless the enclosure is open, and any small animals that may try to enter. The 3R enclosure is a very common type of outdoor enclosure, and is a minimum standard in most building specifications if the transfer switch is to be placed outdoors.

Type 4X Enclosures are rated for both indoor and outdoor use, have all of the same protection of a Type 3R enclosure, and are also able to withstand rusting and corrosion. These types of enclosures are often used near the ocean, or in very humid locations. Enclosures that are in these sort of environments can quickly rust due to blowing salt water, and heavy moisture content in the air.

It is important to be sure the transfer switch in a given system has the proper enclosure to protect the transfer switch from whatever elements it will be exposed to. If installing a transfer switch for a project, be sure to check the building specifications, as it will typically be written in what kind of enclosure the project calls for. On top of that verify with the entity that designed the electrical system as to where the transfer switch will be placed in order to avoid and issues with the transfer switch enclosure.