Magnetic Blowout in DC Applications

Some HVDC (High-Voltage DC) load rated Electromechanical Contactors feature a permanent internal “blowout magnet” to help “blow out” or break the electric arc formed while the contacts are opening. This will allow contacts to be rated at a higher DC rating and help increase electrical life at a rated load. Although there can be arc upon closing of the contacts, the arc is much larger upon opening.

Some Limit Switches, Automotive Relays, & Power Relays (over 2 amps) can also have this feature.

The magnetic blowout feature is listed on the Digi-Key website under the “Features” filter. This is if the datasheet lists the product with magnetic blowout. However, datasheets may not always list this feature even if the product offers this. If the output is polarized, it most likely offers this feature.

Generally speaking, an electromechanical contactor with a polarized DC rated output can handle current in both directions but only while the contacts are closed, the polarization has to be correct only upon opening of the contacts. If the polarity is incorrect upon contact opening, it will have a reverse benefit effect enlarging the arc causing accelerated damage to the contact surface and create a possible hazard. An AC load device typically does not need a blowout magnet, as the arc is inherently self-extinguishing since the AC waveform crosses zero while the contacts are opening.

Viewing the Contact Rating Life Cycles at a specified test condition, or Make Break Chart in the datasheet may help you decide which product is right for you.

Some datasheets also list the contact make/release time including arc duration:

Electromechanical Magnetic Blowout Applications:
Electric Vehicles
-To speed up high-voltage DC charging and safely shutoff and break current loads

Renewable Energy and Power Inverters
-Solar and wind for safe switching and to break DC loads during inverter process as well when moving stored energy to and from batteries

Manufacturing and Inudustry
-Energy storage or backup systems, UAV or fleet management, or even used in systems to charge forklifts

Notes from Sensata:
All Sensata Gigavac contactors can carry current in both directions on closed contacts. The term “bidirectional” refers to the ability to switch/open while current is flowing. Most Sensata Gigavac contactors are designed and optimized to open current that is flowing in one direction to provide maximum current breaking capability. We do have contactors that are designed to be bidirectional. This is generally accomplished by changing the orientation of the internal arc blow-out magnets.

The power terminals on polarized contactors are labeled positive (+) and negative (-). This designation is intended to guide in the proper installation of the product. Most contactors have internal arc blow-out magnets which are used for breaking loads (opening while current is flowing). This is especially important for breaking loads of 48V and higher. The blow-out magnets are designed for a specific direction of current flow. Incorrect installation can limit or negate the benefit of the blow-out magnets and may decrease the life of the contactor significantly. In applications where the system voltage exceeds 48V, care should be taken to install the contactors in the proper direction of current flow. The contactor should be installed so that current flows into the positive (+) terminal and out of the negative (-) terminal.

If bidirectional contactors are used, the main power terminals can be connected in either direction. The internal blowout magnets are configured to break the arc that forms regardless of which direction the current is flowing.

Sensata - Application Note: Direction of Current Flow for Contactors

See also:
Relay Features
Component Options for Relay Coil Surge Suppression
Relays, Switches, and Electrical Contacts in General