IEEE 80 takes as its limit the current that most people withstand without ventricular fibrillation. According to Dalziel, that limit is 0.116 divided by the square root of the time in seconds for a 50 kg person, and 0.157 divided by the square root of the time for 70 kg. It holds for 50 Hz or 60 Hz and for durations from 0.03 s to 3.0 s.
Short answer. IEEE Std 80-2013 does not protect against every electrical sensation, but against ventricular fibrillation, which is the most dangerous consequence of current through the body. For 50 Hz or 60 Hz it adopts the Dalziel formula: the tolerable current is 0.116/√t amperes for 50 kg of body weight and 0.157/√t for 70 kg, with t the shock duration in seconds. The shorter the fault, the more current the body can tolerate.
What effects current has
Current produces different effects depending on its value, duration and frequency. Clause 5.2 orders them from lowest to highest current: perception, muscular contraction, loss of consciousness, heart fibrillation, nerve blocking of breathing and burns. What happens on the ground during the fault and how a person becomes exposed is explained in electric shock from a ground fault in a substation.
| Current | Effect described in IEEE 80 |
|---|---|
| 1 mA | Perception threshold: a slight tingling in hands or fingers |
| 1 mA to 6 mA | Annoying, but the person can usually let go of the object they are holding |
| 9 mA to 25 mA | Can be painful and make it hard or impossible to let go; with higher currents, muscular contraction can make breathing difficult |
| 60 mA to 100 mA | Ventricular fibrillation, cardiac arrest or inhibition of breathing can occur, with injury or death |
Muscular contractions disappear when the current is cut, unless they are so severe that breathing stops for minutes. That is why the guide focuses on the fibrillation threshold: if the design keeps the body current below that threshold, serious injury or death can be avoided.
The effect of frequency
At 50 Hz and 60 Hz the human body is very vulnerable. Clause 5.1 states that it tolerates somewhat more current at 25 Hz and approximately five times more in direct current. At frequencies of 3000 Hz to 10,000 Hz it tolerates even more. The guide is limited to power frequencies of 50 Hz to 60 Hz. Body resistance and the circuit formed by the feet or hand with the ground are in body resistance and the accidental ground circuit.
The duration formula
Dalziel related the current that does not produce fibrillation to the energy absorbed by the body: S_B = I_B² · t_s. From this comes the tolerable body current:
I_B = k / √t_s
where k is the square root of the energy constant S_B. It is assumed that 99.5 % of people withstand without fibrillation the current and time given by the formula. According to Dalziel, the energy that 99.5 % of people of about 50 kg survive gives S_B = 0.0135 and, with it, k = 0.116. For 70 kg the guide adopts S_B = 0.0246 and k = 0.157. The 1961 edition used 0.0272 and 0.165, values that later research replaced.
Since the formula is based on tests from 0.03 s to 3.0 s, it is not valid for very short or very long durations. With it these values are calculated:
| Shock duration | 50 kg (k = 0.116) | 70 kg (k = 0.157) |
|---|---|---|
| 0.1 s | 367 mA | 496 mA |
| 0.5 s | 164 mA | 222 mA |
| 1.0 s | 116 mA | 157 mA |
The 50 kg values at 1 s and 0.1 s are the ones the standard itself cites. The rest result from applying the same formula.
Which of the two weights to use
IEEE 80 allows 0.157 to be used only if the average weight of the exposed people can be expected to be at least 70 kg. Those conditions generally occur at sites without public access, such as fenced switchyards. Outside the fence it must be assessed whether the 50 kg criterion applies.
Which time to take
Since the tolerable current drops as time increases, choosing the duration is a design decision. The guide points to two options: the clearing time of the primary protection or that of the backup protection. With the backup time the safety margin is larger. There is an argument in favor of the primary time: it is unlikely that a relay failure coincides with all the other adverse conditions of an accident.
In addition, studies by Biegelmeier and Lee indicate that the heart is more susceptible to fibrillation when the exposure approaches the period of a heartbeat, and that the risk is lower between 0.06 s and 0.3 s. That is an added incentive for clearing times below 0.5 s.
Comparison with other curves
It is not the only model. Ferris proposed in 1936 a fibrillation threshold of 100 mA. Biegelmeier proposed two thresholds for a 50 kg adult: 500 mA if the shock lasts less than a heartbeat and 50 mA if it lasts longer. His Z-shaped curve has a limit of 500 mA up to 0.2 s and drops to 50 mA from 2.0 s. Clause 6.3 states that, between 0.06 s and 0.7 s, the tolerable current of the Dalziel equation is lower than that of the Biegelmeier Z curve. The guide bases the calculation of tolerable voltages on Dalziel.
A note on reclosing
In modern practice it is common to reclose after a ground fault. A person can receive a first shock without permanent injury. An instantaneous automatic reclose can cause a second shock less than 0.33 s after the start of the first, and that second shock is the one that can produce a serious accident. With manual reclosing the probability is lower, because the interval is usually considerably longer. The cumulative effect of several close shocks is not fully evaluated, but the guide accepts adding the individual durations as if they were a single exposure.
We carry out your grounding study
We define with you which clearing time and which weight criterion apply to each area of your substation, and with them we calculate the tolerable voltages to verify your grid. To estimate them with your data you can also use our free grounding resistance tool. See the engineering design and studies service or write to us through contact.
Related guides
- IEEE 80: Safety in Substation Grounding
- Electric Shock in a Substation: How and Why It Happens
- Fault Clearing Time and Grid Safety
- Step and touch voltage: difference and IEEE 80
- Automatic reclosing 79 (RREC): what it is and how it works
References
- IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding: clauses 5.1 (effect of frequency), 5.2 (magnitude and duration), 5.3 (fast clearing), 6.1 to 6.3 (duration formula, alternative assumptions and comparison with the Biegelmeier curve) and 6.4 (note on reclosing); equations (6) to (9).
- C. F. Dalziel and W. R. Lee, studies on fibrillation current, cited in IEEE Std 80-2013.
- W. Biegelmeier and W. R. Lee, body current versus time curve, cited in IEEE Std 80-2013.
Frequently asked questions
What current can kill a person at 50 Hz or 60 Hz?
IEEE 80 states that people are very vulnerable at these frequencies and that a current of about 0.1 A can be lethal. Between 60 mA and 100 mA, ventricular fibrillation, cardiac arrest or inhibition of breathing can occur.
When is 0.157 used instead of 0.116?
The value 0.157 corresponds to a 70 kg person and can be chosen if the average weight of the population is expected to be at least 70 kg. The guide notes that these conditions usually apply in places without public access, such as fenced or walled yards. For areas outside the fence, it must be assessed whether the 50 kg criterion is more appropriate.
Why does automatic reclosing affect the exposure time?
Because an instantaneous reclose after a ground fault can cause a second shock less than 0.33 s after the start of the first, before the person recovers. The guide allows the effect of nearby shocks to be considered by adding the individual durations as if they were a single exposure.