Calculators
Calculate touch voltage under fault
Under fault the enclosure is not at earth potential but at the voltage drop across the return path. That voltage stands until disconnection. It is the reason disconnection times are limited at all.
Result
- Touch voltage
- —V
- Assessment
- —
- Largest permissible resistance in the return pathAt this fault current.
- —Ω
- Use of the limit
- —%
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A guide, not a design to standard. We give no warranty for the correctness of the results. The applicable standards and case-by-case verification govern.
The grey values are an example. Simply enter your own.
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Calculating is the easy part.
A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.
Method and questions
Touch voltage
What voltage stands on the accessible part under fault.
Discuss a projectWorked example
The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.
Inputs
- System earthing
- TN (return through the protective conductor)
- Fault current
- 1,000 A
- Resistance in the return path
- 0.083 Ω
- Permissible touch voltage
- 50 V
Result
- Touch voltage
- 83 V
- Assessment
- The limit is exceeded. Disconnection has to occur within the required time.
- Largest permissible resistance in the return path
- 0.05 Ω
- Use of the limit
- 166 %
Formula and standards
Formula
- U_B = I_fault · R_return
- TN: R = protective conductor resistance
- TT: R = earth resistance of the installation
- R_max = U_permissible / I_fault
Assumptions and standards
- The calculation gives the voltage on the faulty equipment against reference earth while the fault persists.
- The permissible limit depends on the location. Special environments such as wet areas, agricultural premises and medical locations have lower values.
- The limit alone is not enough. If exceeded, disconnection has to occur within the required time. How quickly depends on the magnitude of the voltage.
Why is touch voltage a bigger issue in a TT system?
Because the fault current returns through earth, and earth resistance is orders of magnitude higher than a protective conductor. A few hundred milliamperes across ten or twenty ohms already gives a dangerous voltage. That is why a residual current device is practically indispensable in a TT system.
Why is a smaller protective conductor not simply acceptable?
Because a smaller protective conductor has more resistance, so the touch voltage rises, proportionally at the same fault current. At the same time the fault current falls, because the loop becomes more resistive, and disconnection takes longer. Both effects work in the same wrong direction.
Is a brief excursion harmless?
Not as a blanket rule. The hazard depends on magnitude and duration together. That is why the rules couple them. The higher the touch voltage, the shorter the disconnection time must be. A high voltage gone in milliseconds is something different from a moderate one that stays.
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