Why DC Circuit Breakers Can’t be Used in an AC Circuit and Vice Versa?
Both AC and DC are electrical energies having the same heating effects for the same RMS values but there are multiple differences between AC and DC with different parameters.
AC Breakers in a DC Circuit
For instance, the AC voltage and current may have different effect in the circuit other than the DC with the same level of voltage. For this reason, An AC circuit breaker can’t be used for circuits. Similarly, DC circuit breakers can not be used for AC circuits. Let’s see the following facts behind the scene.
We know that AC needs more insulation than DC having the same level of voltage. This way, the insulation material used in a circuit breaker and its contacts may react differently facing the opponent voltage other than the rated voltage.
In case of AC, the sine waves of current and voltage reaches to the zero about 60 times per second (in US where frequency is 60Hz while 50 time in UK and EU where f = 50Hz). In other words, AC voltage and current cross the zero after each half cycle due to frequency and they change the direction and magnitude at time interval.
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DC Breaker in an AC Circuit
In case of DC, zero crossing is not possible as there in no frequency in DC voltage and current as DC is a constant value i.e. DC voltage and current won’t change the direction and magnitude (it flows in one direction and can’t changes the polarity).
This way, DC will melt the breaker contacts faster than DC as DC is constant while AC changes it direction and magnitude 50 or 60 times a second.
Why Arc in DC is stronger than AC in Breaker Contacts?
When we break a DC circuit via DC circuit breaker, the electrons flows are steady state and want flow through the contacts as the applied voltage pushes them forward, so the created arc is stronger as compare to the AC as DC flow of electrons have momentum.
In case of AC, the applied voltage and current are not steady state i.e. the flow of electrons are relaxed as they back and fourth changing the direction and amplitude continuously. In other words, the firstly go to the peak value and go back to the zero in the first half cycle (+ve) and then go to the peak and zero again (respectively) but in the opposite direction (-ve). This vibration distort to create the momentum, so the arc created in AC is weaker than DC voltage.
Keep in mind that you won’t be able to feel the zero crossing of AC as the effect of changing of AC is too much fast even you can’t assume that the AC at zero value at any instance. Keep again in mind that AC is more dangerous than DC.
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These are the reasons you should not use DC circuit breaker for AC and vise versa.
Is it still possible to use DC breaker on AC and AC breaker on DC?
If you need a favor (you did) and find a way to use AC breakers on DC, You can, but you may not.
Before going to do so, you should read all the instruction, circuit breaker nameplate data printed on it especially the voltage and current rating for both AC and DC.
There are some circuit breakers which can be used for both AC and DC with limitation. For instance, a 240V AC, 30A circuit breaker can be used for 48 to 60V DC. Similarly, a 110V AC circuit breaker can only be used for 30V DC.
To use an AC circuit breaker in a DC circuit, the proper size of circuit breaker should be used. This is because the breaker do the following two jobs when overcurrent fault occurs on the connected circuit.
The current sensing system sense and identify the overcurrent which leads to separate the breaker contact to break the circuit from power supply.
The circuit breaker then quench and distinguish the arc created safely during the contacts separation without damaging the breaker and contacts. This is only possible when we use correct size of wire and breaker according to the NEC and IEC standards and regulation for wiring installation.
- Related Post: Why Do Electronic Circuits Use DC Current instead of AC?
kA Capacity when using AC and DC Circuit Breakers
kA rating of a circuit breaker must be consider while using it for AC and DC circuits. It is the breaker rating in amperes based on the peak value (breaking capacity rated in RMS value of currents) which shows the making abilities in case of surge and transient voltages for very short time.
For this reason, the voltage and current rating reduced to the half of the AC voltage and current. For instance, the 10kA AC will reduce to the 5kA in case of DC circuit. Now, if a fault of 5kA occurs on DC line, the breaker will act as short path due to to melted contacts and over connected devices may damage which even leads to hazardous fire and electric shock.
Due to the above reasons and facts, we don’t recommend to use AC circuit breaker for DC and vise versa.
Note: The author will not be liable for any losses, injuries, or damages from the display or use of this information or if you try any circuit in wrong format. So please! Be careful because it’s all about electricity and electricity is too dangerous.
Disconnect the power before replacing, repairing, troubleshooting, maintenance and installation electrical appliances and equipment other wise, it will kill you and you won’t be able to ask that who killed me – Current or voltage? Also, you won’t be able to ask which one roast me AC or DC? So please stay safe.
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