Last edit: 04/07/2025
Everyone knows that current is dangerous, but how can you tell what the dangerous threshold value is?
Electrical safety standards come to our aid. It is the IEC 60479-1: Effects of current on human beings and livestock series of standards. The standard deals with the dangerousness of direct and alternating current. First, let’s start by distinguishing between the two types of current and why they are dangerous to the human body:
- Alternating Current (AC), is used everywhere: in the home environment, or even in offices or factories. It has a sinusoidal waveform, that is, the voltage and current rise and fall continuously and smoothly, with a frequency of 50 Hz (60 Hz in North America and a few other nations). We now come to its dangerousness. The heart has its own rhythm of operation, induced by electrical pulses with a frequency of about 1 Hz. When one is subjected to an electric shock, a current with a frequency of 50 Hz circulates through the body. In this case, the heart begins to function at the mains frequency and no longer pulses at its natural frequency. In other words, the heart’s frequency is overpowered by that of the current, altering its rhythm, leading the heart to vibrate instead of pumping (cardiac fibrillation).
- Direct Current (DC), on the other hand, is found in batteries, electric vehicles, photovoltaic systems and many other applications. It has a pulse, is less likely to lead to fibrillation, but increases the risk of a (cardiac arrest), as it does not alter the rhythm of the heart, but keeps it contracted. Direct Current is considered less dangerous, as it causes damage at higher voltage values than Alternating Current. In this regard, one is reminded of the famous “War of Currents,” or the rivalry between Thomas Edison and Nikola Tesla in the late 1800s.
The curves for the current hazard through the body
To graphically show the danger of current, graphs have been created that allow us to identify the values of currents in relation to the times in which they can cause more or less serious damage to the body. There are graphs for both alternating and direct current.
These charts are divided into different zones, with which different effects are associated, such as:
Zone 1: no reaction (<0.5 mA).
Zone 2: no major physiological effects.
Zone 3: reversible pathological effects, such as muscle contraction.
Zone 4: dangerous area for probable fibrillation.
The zones considered dangerous are zone 3 and 4. In turn, zone 4 is divided into 3 other subareas:
- Under C1 curve : no reasonable fibrillation.
- Under C2: risk of fibrillation 5%.
- Under C3 : risk of fibrillation 50%.
Here at the side is the graph for AC Current.
The table for how dangerous if a DC current is here at the side. Looking at the table for Direct Current, we can see that Direct Currents are 4 times less dangerous, compared to Alternating Current (for values as per the table). Note for example how the C1 curve starts around 40 mA in Alternating, while in Direct starts around 160 mA
The Electrical Safety Curve (Current)
Regarding the danger for people of alternating current, the precise limit above which the current, as a function of time, is dangerous was decided in the 1980s, based upon the graph discussed above.
This is indicated by the dashed curve in the image at the side, and called the Safety Curve for Alternating Current.
The curves shown in the graph, to the left and right of the green curve are curve “b” and curve “c1” in the previous graph, respectively. This curve is important for the setting of the protections designed to protect people from indirect contacts. From this curve, it can be derived, for example, that with a current of 200 mA, the maximum circuit opening time is about 150 ms.
The Electrical Safety Curve (Voltage)
The voltage safety curve represents the voltage values in relation to contact time (Indirect contact), for which a person can have a fibrillation. A hand-feet path is considered. The starting point for going from the current safety curve to the voltage curve is the resistance of the human body, which depends upon the voltage, as shown in the table.
The standard estimates the shoe resistance plus the gound resistance (REB) to be 1000 Ω, for normal conditions; instead, for critical conditions (stables, hospitals) 200 Ω. However, it remains to add up the body resistance (RB), which, as it can be seen from the table the higher the voltage, the lower the body resistance. Based on that table, it is possible to define a relationship between different contact voltages and the maximum time a person can be subjected to that voltage, without running a significant risk of fibrillation.
For example, if the voltage is 75 Vac under normal conditions we take a resistance of 1625 Ω (1000 Ω to ground REB+ 625 Ω path hand body foot RB, we can calculate a current of about 46 mA. Looking at the graph we have a maximum time of about 1s.
Considering 92 Vac as the contact voltage, we conventionally take 600 Ω as the “normative” resistance for a current path from the hand, to the feet.
Thus, the graph for the safety curve in voltage starts from the one in current: knowing the safety curve in current and having a resistance of the human body, it is possible to calculate the voltage according to Ohm’s first law:
V = R * I
TN system and maximum protection opening time
The drawing shows how the Exposed Conductive Parts (ECPs) are grounded in a TN (earth-neutral) system, which is normally used in industrial settings, as opposed to the TT (earth-earth) system, used in domestic premises.
In a TT system, the ECPs are connected to the “dirty ground” and, through this, they are connected to the neutral point of the transformer supplying the electrical system. In this case, the fault loop includes not only copper wires but the earth itself.
In a TN system, on the other hand, the ECPs are connected to the neutral of the transformer. In this case the fault loop includes only copper wires. For this reason, a ground fault with negligible resistance generates very high currents.
From the above, it can be understood that the connection of an electrical equipment to a PE cable has the only aim of creating a fault loop, i.e., an electrical circuit that is normally not live. In other words, no current normally flows through the PE cable.
In the event of a earth (ground) fault, however, this circuit is activated and a very high current flows through it, in the case of a TN system. This current activates the magnetic or differential protection, which opens the circuit.