What do you mean, “Is it safe?”
Posted by Mitch Randall on March 01, 2015
All of the wireless charging technologies that you have likely heard of, except Open Dots, is based on electromagnetic induction. This is where an electromagnetic field is produced by the pad that extends into the surrounding space. When a device to be charged is placed into this region, it converts the energy contained in the fields in this region into useable electric power.
No doubt most people do not understand how this works, nor do they care, but we can be sure it is subject to regulations so that we can be certain it’s safe, right?
Well, the answer is, I do not think so.
In fact, I have studied the question of safety of electromagnetic induction in great detail. I was surprised to find that the regulatory safety limits – the “uniform field limits” - used by the industry are not the limits that should be applied. The correct limits would be more difficult if not impossible to meet than those I have seen the industry inappropriately apply. That means that when they say it is safe, it actually may not be.
To explain what I mean I am going to introduce Nigel, an unlucky test subject. The “uniform field limits” apply well to the situation in the below picture, where Nigel is standing
under a high voltage power line. As far as Nigel is concerned, the electric field lines are more or less vertical and uniform. We estimate that the field strength is about 3000 Volts per meter, and because he is about 2 meters tall, his body sees a potential of about 6000V.
You might wonder why Nigel is still standing, because if you connect 6000V from your head to your toes, it seems like something very very bad would happen to you.
The answer is that because there are no wires, Nigel is an open circuit. What happens is that a tiny electric displacement current flows upward in his body, but at his head it has nowhere to go. This causes a charge to build up on Nigel’s head until it cancels the field and no further current flows.
This is the basis of the “uniform electric field” limits imposed by the government. Here we see that a 6000V field causes only tiny displacement currents to flow in Nigel’s body. You can think of it in simple terms: Nigel is an open circuit. There is no path for current to flow out of his head and back into his feet to cause him harm. Therefore, very large uniform electric fields can be permitted because they cause only a tiny displacement current.
The problem is that the fields used in inductive charging are not uniform. In fact, they are specifically designed to cause currents to flow in the receiver (device). To see what I mean, consider Nigel standing between the two coils of one particular inductive charger, Witricity.
Let me explain why this is important. In inductive charging, the electric field lines form circles very much by design. The purpose is to cause considerable current to flow in the receiver in the same circular fashion.
For Nigel, this means that he is now a closed circuit. If currents flow in him, they do not run into a dead end and build up an opposing charge to cancel the field. Instead currents are induced to flow around in loops inside his body.
Believe me, if the circular field shown in Nigel’s body were the same 3000 Volt per meter magnitude as the power line example, Nigel would quickly be roasted.
I hope I have demonstrated that the uniform electric field limits should not be the limits used to validate the safety of inductive chargers. These limits are for use in situations where the human body is essentially an open circuit – a light switch that is off. But when the field lines are circular like they are in inductive chargers, the human body is a closed circuit – a light switch that is on.
I have never once seen or heard any admission by inductive charging companies of the fact that the uniform field limit is an inappropriate standard to use to validate human safety. Are these devices safe? It is not known. Are safety regulations being applied appropriately? I don’t think so.
On a more positive note, one of the reasons I am a big fan of the Open Dots wireless power technology is because it does not use electromagnetic fields. As such, I can say with complete certainty that it is guaranteed to be safe. With Open Dots wireless power technology, there is absolutely no chance of long-term negative health effects. Period.