Shock

This is a great followup and use case for a home energy monitoring system, something I wrote about on the blog last week.

My new ETS was broken, but how?

The electric thermal storage (ETS) unit mentioned in that post stopped doing what its supposed to.

When we had it installed I didn’t tie it into the PLC because of time commitments, so the contractor put a programmable thermostat in as the charge controller. If its within the right time AND the temperature in the room is lower than the setpoint it start charging. The internal charge thermostats turn off the charging for the three rows of bricks.

This weekend we noticed that the unit didn’t stop when it was supposed to. We also had a minor flood in the basement this weekend so I put the thermostat in manual to just get the heat going to dry it out, so we initially thought that was the problem. Turns out, no, it was something bigger.

Before we even had to break out the control schematics, we were able to figure out what was likely going on.

SCADA as a troubleshooting tool

By using the heating screen on the SCADA, the backbone of the Home Energy Monitoring System, we were able to troubleshoot the system without exposing ourselves to 208V. All the covers could stay closed.

To do that we monitored the power output from the unit modifying the various controls that would turn the unit off. When the only action that would shut it down was the breaker we took to google.

Turns out that these units have a failure mode that the charge controlling relay “sticks”, and that is where I will get looking today to find out if there is anything that I can do to fix it.

Moral of the story

This isn’t about the home energy monitor, basement floods, or even ETS; this is an example of using properly calibrated (and tested) SCADA and troubleshooting procedures to identify the likely culprit to an issue before opening a panel.

CSA Z462 and NFAP 70E have a provision where you don’t need an Energized Work Permit for troubleshooting, because troubleshooting isn’t “work”. While troubleshooting energized isn’t “wrong” that doesn’t mean that its safe.

When you have input during a control system or SCADA upgrade, include provision for troubleshooting telemetry and using a platform like VTScada the cost per IO (in SCADA hardware and licenses) is minimal when compared to a safety incident.

This same telemetry can be used for advanced asset management which has other business applications.

Don’t expose yourself to energized parts, unless there really is no other option.

A little while ago, I talked about two important components of electricity: voltage and current. Understanding both is key to understanding how to keep yourself safe around electricity.

Electric Shock_thumb[1] — You don’t want to be this guy.

There are two big hazards to look out for when working with and around electricity: electric shock, and arc flash. Electric shock happens when electric current passes through the human body, and arc flash happens when you have a very large, explosive discharge of electric energy in equipment. Today I’m going to talk about electric shock and the human body. Specifically, I’m going to talk about what levels of electricity are safe, what levels can hurt you, and what levels can kill you.

Electric Shock and the Human Body

Electric shock is the physical act of electric current passing through the human body. The effects of electric shock can range from a slight tingling sensation, to immediate death. This handy chart is a good reference for how different types of current (AC and DC having different safety thresholds):

Effects of electric current entering the human body for men and women

As you can see from the chart, 60 Hz AC is much more dangerous than DC or 10 kHz AC. This is because of the way the body works: to get a muscle to move, the brain has to send a tiny electrical signal along the nervous system to the muscle. The most important signals that we’re concerned with are the ones being sent to the heart muscles: most people have a heart that beats at a frequency of about 60-100 Hz. If you want to test this, place your index and middle finger of your left hand on your neck, under your jaw and directly below your ear, then count the number of beats or pulses you can feel in a minute.

Why is low frequency current dangerous to the heart?

Because the heart beats at a fairly low frequency, low frequency current like the kind found in modern day power systems (60Hz in North America) is very dangerous because it can cause heart fibrillation. The heart is made up of a group of muscles that all have to work together to pump the blood through your body. Heart fibrillation is the medical term for when these muscles are no longer working together: your heart is off beat, and “flutters” or beats weakly but way faster than normal. This weakly beating heart can’t pump blood through your body properly. The blood won’t get to the lungs for oxygen, and the heart will quickly run out of oxygen and stop. The is known as cardiac arrest, or a heart attack.
DC and high frequency AC electricity are still very dangerous, but because they apply current continuously (or the current pulses so fast that your body thinks it’s applying current continuously), it takes more current to override your body’s natural electrical signals. Kind of like how your eyes can only see the world at 60 frames per second, and you just don’t recognize faster speeds.
This is also why a defibrillator works: the paddles zap you with a big dose of DC current to stop your heart, but your brain is still sending signals telling the heart to operate like normal. If the heart is fluttering uncontrollably these signals from the brain wouldn’t do much, but if the heart is stopped it just starts up again like business as usual.

Effects of electricity on other muscles

In addition to the nearly immediate death caused by electric current interfering with the heart, electric current can also activate your muscles, causing them to contract or flex. With DC, this usually causes one burst of muscle movement which could throw you away from whatever you’re working on. AC, on the other hand, causes your muscles to continuously flex over and over again, which could cause you to spasm out of control. In both cases, the point where you lose control of your muscles is called the “Let-Go Current Threshold”. For example, if you were shocked because you grabbed a wire, the current will cause your hand to grip the wire tightly and you won’t be able to let go. The longer you are exposed to electricity, the more damage it causes, so this is clearly a very bad scenario. With both AC and DC, working from an elevated position (like on power lines) adds a risk of a fall injury if you lose control of your body from that high up in the air. If the muscles that move your limbs away from your body get activated first, you could throw yourself out of the bucket in the truck.

Electric Shock Burns

One last thing to worry about are electric burns. Just like a wire will break down due to heat when too much current moves through it, your body will burn during electric shock. The longer you have current moving through your body and the higher the amperage, the more severe the burns will be. One of the most dangerous parts of electric shock burns are that they burn you up from the inside out. Even after what seems like a non-fatal electric shock, the tissue inside your body could have been burned and scarred or killed. Dead and damaged tissue can all sorts of negative effects, including organ failure and amputation.

How can you prevent current from entering your body?

The human body has its own natural way to prevent current from entering it: skin resistance. The resistance of dry human skin can be as high as 100,000 ohms. This resistance gets dramatically reduced if the skin is wet

This picture is pretty self explanatory.

(sweat is a salty liquid, and water mixing with the salt on the surface of your skin will drastically reduce skin resistance as well). The resistance of the human body can also be lowered by any breaks in the skin (like a cut that hasn’t healed yet). You want to provide the path of greatest resistance to the electricity in order to stay safe, and an open cut is a great, easy spot for current to flow into your body.
Another thing to keep in mind is that the breakdown voltage of human skin is 500 volts. This means that, at 500V and higher, the outer layer of the skin is destroyed by the high electrical energies. This results in drastically lower skin resistance.
And, as always, be sure to wear appropriate personal protective equipment (PPE) for the job. PPE is designed to keep you safe, and that extra bit of insulation could save your life.
So, in summary: stay dry, stay covered, and wear your PPE!
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