10 Safety Tips on Electrocution for the Network Engineer

Short version – I’d like you to read just these if nothing else.

  1. A data centre uses a lot of electricity. It’s dangerous.
  2. Just in case you didn’t pay attention – Electricity is really dangerous. Right, can we move on ?
  3. You should be scared of being electrocuted. That will keep you safe.
  4. When electrocution happens, the muscles in your limbs contract. This causes arms and hands to wrap around or clench objects.
  5. Someone who is being electrocuted can’t let go because of this
  6. If you touch a person being electrocuted, you will get an electric shock too, and may also be injured.
  7. The BEST thing to do is to turn the power off.
  8. Know where the power kill switch is for Data Centre. Or at least the breaker/switch for the area that you are in.
  9. Don’t work in the Data Centre alone.
  10. Learn resuscitation. People who have been electrocuted are likely to have breathing problems and heart failure.

People shouldn’t die at work. Think seriously about going home everyday and what you can do to make that happen.

Basics of Electric Shock

It takes very little current to cause damage to the human body. Many people don’t understand why sometimes electrical injuries vary from person to person.

Studies show that the electrical resistance of the human body varies with the amount of moisture on the skin, the type of body that the person has and the applied voltage. The typical hand-to-hand resistance of the human varies 500 Ω and 600 kΩ, depending on the conditions although other sources say 1KΩ to 100KΩ – in other words, there is not precise answer. Higher voltages have the capability to break down the outer layers of the skin, as burns, which can reduce the overall resistance value. UL uses the lower value, 500 Ω, as the standard resistance between major extremities, such as from the hand to the foot. This value is generally considered the minimum that would be encountered and, in fact, may not be unusual because wet conditions or a cut or other break in the skin significantly reduces human body resistance.

To summarise, you can’t really predict what voltage/current will really hurt, damage or kill a person. It’s too variable.

The Effects of Current on the Human Body
Current Effect
1 mA or less No sensation, not felt
More than 3 mA Painful shock
More than 10 mA Local muscle contractions, sufficient to cause “freezing” to the circuit for 2.5 percent of the population
More than 15 mA Local muscle contractions, sufficient to cause “freezing” to the circuit for 50 percent of the population
More than 30 mA Breathing is difficult, can cause unconsciousness
50 mA to 100 mA Possible ventricular fibrillation
100 mA to 200 mA Certain ventricular fibrillation
More than 200 mA Severe burns and muscular contractions; heart more apt to stop than to go into fibrillation
More than a few amperes Irreparable damage to body tissue

The typical current rating of a typical AC power socket does vary from country to country but it’s at least 5 AMPS. Yes, that’s more than enough to kill. 

The type of injury caused by electrocution will also vary where the current passes through the body. In the worst case, an electrical current of about 10-20 milliamps applied to the back of a persons head is enough to kill. A current passing from hand to hand (reasonably common) has a much higher risk of impacting the heart. A current that passes within a limb is less likely to cause severe injury to the heart or nervous system.

Of course, time is also a factor. The longer the current flows through the human body, the more damage is caused and severe injury is more likely.Also,longer exposure increases the chance of electrical burns and irreparable flesh damage.

Loss of Function

If you survive an electrocution, then you have a high probability of permanent damage to limb or body. The current flowing through the body can cause permanent nerve damage,

DC Power is more dangerous.

Also DC power is much more dangerous than AC power because it tends to use low voltage and high current. It’s the current that is more likely to damage the body thus causing injury and death.

High Voltage is Dangerous by Proximity

High voltage is still dangerous because it’s can create a lot of current, but also because high voltage can jump/spark large gaps. When you are close to high voltage sources, say 1 kilovolts or more then think about whether you are path to ground.

That’s why the power distribution systems are stored in separate rooms in the data centre. To keep you safe and prevent you from getting close enough to hurt yourself. Don’t go in there unless you have to.


People who have been seriously electrocuted are likely to have breathing difficulties and possibly heart failure. You should learn resuscitation because you really need the hands on training to know how to do it right. That’s what I found when I did it – it seemed easy but it was hard to do.

Basics Tips – for full advice I recommend reading the Safety First Web Site

  1. Check that you are safe and able to help the victim. You can’t help the person if you are also incapacitated.
  2. Disable the power to the building or area. Insulate the area with mats or other material if you can’t do this.
  3. Check patient is conscious.
  4. Call for help.
  5. Check the casualty’s Airway, Breathing and Circulation (ABC).
  6. Check the airway / assess breathing. If no breathing consider moving to recovery position and clear obstruction. Victim may have swallowed tongue.
  7. Administer CPR if no breathing.

Ground Loop Breakers

I don’t know the proper name for these, but it’s possible that a safety ground loop breaker will save you. But they aren’t that reliable. I been buzzed a few times and ground loop didn’t go off. I guess I was not very conductive on those days.

The EtherealMind View

I’ve been electrocuted a few times when I used to work on circuit boards and power supplies. Lets just say a) I was lucky b) it was not fun. These days a lot more consideration is given to practical safety and many people no longer consider that a computer device can be a risk. Vendors have made a lot of effort to make their equipment safe and your data centre electrician will also have safety plans and preparations.

That said, I’ve met people with serious electrical burns that will never heal or damaged bodies. Gruesome.

Take some care to keep yourself safe.


I have nothing to disclose in this article. My full disclosure statement is here

  • http://www.facebook.com/benjamin.henriksen.7 Benjamin Ginsommer Henriksen

    Double post

  • http://www.facebook.com/benjamin.henriksen.7 Benjamin Ginsommer Henriksen

    Great post Greg, a lot of people – definitely myself including, doesn’t take this stuff serious enough. My eye-opener came a few months ago when i nearly fell off a pretty high ladder, efter getting zapped by a metal patch panel. Small *zap*, but high ladder..

    Edit: Sorry for double post

  • Josh Barron

    I’ve been hit a few times working with power, mostly just at home rewiring an outlet that I thought was disabled. I followed the map on the breakerbox and didn’t think about doing much testing to the outlet before touching it bare. Learned my lesson quick to never trust the mapping on the box.
    DC power on the other hand, terrifies me. I don’t understand why DC power plants and rectifiers have their connections so damn close to ether, sometimes surrounded by nothing more than a simple piece of black plastic between positive/negative/ground. Low voltage indeed, but hundreds of amps.

    Anyways, good article and something that a lot of data center workers don’t pay enough heed to. The biggest take away is to always be aware of your surroundings.

  • PG

    I believe everyone should experience a small shock to make sure that they ARE scared of it. Don’t know how to do that safely though :)

    When I was doing training to allow me to enter sub stations, they told us that low voltage AC (eg 240v) will likely kill you, but high voltage will make you wish you were killed. HV tends to throw you away from the source, but you will get horrendous burns and injuries, and can basically get cooked from the inside.

    If you are not trained, stay the hell away from it, distance is your friend.

  • http://twitter.com/paulmwalton Paul Walton


  • http://twitter.com/paulmwalton Paul Walton

    Ground Loop Breakers or Earth leakage breakers are designed to shut off the power and save you in 40ms (miliseconds). As Greg states they can still hurt and are used on general power outlets for tools etc. The UPS protected supply to the switch or server probably doesn’t have one.
    Beware stand alone UPS with batteries in the bottom of racks DC hurts . . Lots!

  • Alan Hinton

    Really important stuff. I used to work on troposcatter radio systems which were incredibley high powered (1 to 10kw INTO the antennae) and we were always directed to work on the radio elements with one hand only (to redcue the risk of crossing the heart). Still do it today out of habit.

  • Pseudocyber

    I heard something somewhere that if YOU ABSOLUTELY HAVE TO …. and I can’t imagine a circumstance where you have to … touch a wire you suspect might be live … use the BACK of your fingers. Then, when you get electrocuted and your hand closes, it the muscles contracting actually remove the contact with the wire, instead of grabbing onto it.

    • http://etherealmind.com Etherealmind

      That’s an old trick that some electricians use when it’s too far to the truck to get the tester. I’ve had several jolts over the years and I’d not recommend this. Hurts a lot.

  • http://twitter.com/Vegaskid1973 Matt Thompson

    For some strange reason, I was drawn to electricity as a young kid. On more than one occasion, I have removed the lightbulb from my bedside lamp and stuck my fingers against the pins. In the UK, that’s 240V of OUCH. I also buggered up my dad’s analog multimeter by sticking the leads in a kettle’s main lead whilst it was plugged in. On that occasion, I was only given a mild telling off as my dad marched off the shop to buy the newer digital model he’d been eyeing up for some time but couldnt justify to my mum!

    Have always attributed it to my youthful inquisitive nature! 😉 Thankfully, common sense now prevails most of the time.

    A few points I would make about this article:

    DC is not more dangerous than AC per se. AC versus DC at the same voltagecurrent are about as dangerous as each other but as you state, in the Data Centre, DC can often draw more current.

    It is common in data centres to find three phase power. If you don’t understand how this works, you can end up with 480V (UK) of potential in a cabinet which has a tendency to melt things.

    As you state, it’s the current that is the killer. You can easily have several million volts passed through your body without risk of death although, in the case of a taser gun, it’s still going to ruin your day.

  • guest

    If your are an electrician or something similar in Germany you have learned the “Fünf Sicherheitsregeln” – “five safety rules” during your apprenticeship. These safety rules are:

    1. “Freischalten” – disconnect / switch off / de-energize
    2. “Gegen Wiedereinschalten sichern” – protect against reconnecting
    3. “Spannungsfreiheit feststellen” – check for de-energization
    4. “Erden und kurzschließen” – ground and shortcut
    5. “Benachbarte, unter Spannung stehende Teile abdecken oder abschranken” – cover energized bordering regions/parts

    And you should better know them by heart and always use them.

  • http://twitter.com/subnetwork Jonathan Davis

    I have to take issue with #4 and 5.

    Growing up as the son of an electrician, I was around electricity a lot. I worked throughout high school with various electrical companies, and left an industrial electrical company to accept my first IT job.

    I’ve been shocked with 110, 208, and 240 AC. I’ve also been in the same room as a 4160V AC service that blew thanks to the EMI and RF not being properly shielded across multiple parallel circuits.

    Never have I been unable to remove my hand from the circuit. There are just as many muscles in our hands responsible for opening the hand as for closing it, and our body’s response is to pull the hand back.

    One additional note, it takes 3,000 volts of electricity to arc 1″. Near is relative when discussion electricity.

    I agree with the premise of the article though. Safety should be the number one concern when it comes to electricity. If you don’t have a solid understanding of electricity, call in the professionals, not only for your personal safety, but the safety of your datacenter and the people who come behind you.

    • http://etherealmind.com Greg Ferro

      I’ve researched some of this material from professional safety manuals. The grip part is mentioned in most electrical safety material. I’ve had to pull a few people of 240V and 500V bus bars and the gripping is a serious problem in sustained injury cases. In short, you are incorrect on that point.

      I agree with the variable nature of electricity and I specifically point out the variables and what makes them different so that people don’t get lazy about WHY its dangerous at differetn times.

  • splat

    DC is not more dangerous than AC! Everything else in your article you got right. Go out to your car and touch both terminals of your battery if you don’t believe me.

    Car batteries supply high current perhaps 100amps for 30seconds. But electric shock depends on human resistance, as you previously stated – ie the current running through us when touching the terminals is extremely low and only detectable if you are very sweaty etc.

    • http://etherealmind.com Etherealmind

      Incorrect. DC current draw is far higher than AC due to use cases. Statistically, more people die from DC electrocution than AC.

      A battery is less likely to cause fatality because of the limited voltage and current draw. But working a data centre with 400V DC circuits is a life threatening situation.

  • Benjamin Maccabee


    High Voltage is Dangerous by Proximity

    High voltage is still dangerous because ***it’s*** can create a lot of current, but also because high voltage can jump/spark large gaps. When you are close to high voltage sources, say 1 kilovolts or more then think about whether you are path to ground.

    • http://etherealmind.com Etherealmind

      Yes, I’m doubtful that MP-TCP will ever take off, changes to the TCP stack are hard (see IPv6 for more information and practical examples ….)