Unplugging DC Without Getting Shocked
Nobody gives a second thought to pulling the plug on common household devices and electronics, all connected to the 120V AC power outlet.
Think how often you see people charging cell phones, iPads, tablets and laptops at the local coffee shop.
To disconnect, many people just give a quick yank on the cord, a typical but not recommended practice as it can damage the plug and the cord. But there’s no real danger of the AC power arcing out of the outlet and giving you an unhealthy ZAP!
However, that’s not the case with DC. Unlike the 60Hz AC (Alternating Current) power which crosses through a zero output 120 times per second (imagine a sine wave) due to the alternating part of its name, DC (Direct Current) is always on. And remember, it’s the current that’s a killer. Which is why accidentally shorting out your 12VDC, 48A car battery with your hands, making an electrical circuit which passes your heart, can be fatal.
Without the built in safety feature of AC, nor the fast response circuit breakers and ground fault interrupt (GFI) installed with all residential and commercial AC wiring, products using DC need to implement additional protections. Data centers and network communications can have equipment running with 360V DC, which requires serious protection.
As an aside – for low power applications such laptop chargers, it’s not a hazardous situation since the AC to DC converter provides protection as well as limits both the voltage and current outputs. To further prevent equipment damage from shorts, rather than personal injury, the cable from the AC to DC converter uses a pin and ring configuration. The outer ring provides a physical barrier to people (especially children) putting their finger inside and completing a circuit. For cell phones, tablets and other USB charged / powered devices, these are also limited at the source as to how much voltage and current they can provide. And the extremely small form factor of the connector, as well as the pin configurations make it practically impossible to accidentally touch the power supply pins. For more information about protection, see our article on Ingress Protection (IP) standards.
For higher amperage systems, plug and cable manufacturers are building intelligence into the physical plug. These typically have a third pin or blade, in addition to the standard two for the DC power and ground. The additional connection provides a make-or-break control function at the plug. This control pin is anywhere from 5% to 20% shorter than the other two pins. These three pins are contained within the cable head housing to provide an insertion guide, as well as key-orientation, to the equipment.
This means that on insertion, the two standard DC pins in the cable head will make contact with their counterparts in the equipment. But until the control pin makes contact, there DC power remains off. Insertion isn’t the main cause for concern, as indicated from this article title. It’s the unplugging or removal of the power cord from the equipment. As the plug is removed, the control pin will break contact before the DC pins decouple from the equipment. As soon as the control pin breaks contact, the power to the DC is shut-off, so there is no current flowing by the time the DC power and ground pins break contact. Without any current flowing, there is no chance for an arc or getting shocked.
A simple third pin and some control logic for early disconnection signaling is all that’s needed for unplugging DC without getting shocked. Of course, this has to be planned into the initial design so that the equipment can accept this type of plug or connector. Please visit Assmann WSW Components for all your connector and custom cable needs.
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