33 — Calculators
Design residual current protection
A residual current device cannot tell a fault from the sum of many small operational leakage currents. Every switched-mode supply, every drive, every screened cable pays in. When an installation trips "for no reason" after an extension, that is exactly the cause.
Result
- Total leakage current
- —mA
- Risk of nuisance tripping
- —
- Share of the rated residual currentA third is taken as the upper bound for trouble-free operation.
- —%
- Leakage current left up to one third
- —mA
- Discrimination with the upstream device
- —
Please fill all fields with valid numbers.
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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.
Worked example
The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.
Inputs
- Rated residual current I_Δn
- 30 mA
- Number of connected items of equipment
- 6
- Leakage current per item
- 1.2 mA
- Leakage current of the cabling
- 2 mA
- Rated residual current of the upstream device
- 300 mA
Result
- Total leakage current
- 9.2 mA
- Risk of nuisance tripping
- Uncritical. The leakage currents stay below one third of the rated residual current.
- Share of the rated residual current
- 30.7 %
- Leakage current left up to one third
- 0.8 mA
- Discrimination with the upstream device
- Ratio met. The upstream device additionally has to be time-delayed.
Formula
- I_leakage = n · I_device + I_cabling
- Rule of thumb: I_leakage ≤ I_Δn / 3
- Discrimination: I_Δn(upstream) ≥ 3 · I_Δn(downstream), plus time delay
Assumptions and standards
- The leakage currents of the equipment are entered here; they come from the data sheets and differ by orders of magnitude.
- One third of the rated residual current is a rule of thumb, not a standard. It allows for a device being permitted to operate from half its rated value upwards.
- For discrimination the ratio of rated residual currents is not enough. The upstream device must additionally be time-delayed — otherwise both operate together.
- Leakage currents add arithmetically only as an approximation. With markedly different frequency content the actual total is lower.
Frequently asked
Why a third and not a half?
Because a residual current device is permitted to operate anywhere between half and the full rated value — that is allowed scatter, not a defect. Loading up to half leaves no margin against a device at the lower end of that range.
What if the total is too high?
Split it. Several circuits each with their own device instead of one large one — that distributes the leakage currents and at the same time limits what disconnects under fault. Choosing a higher rated residual current is the poorer route where personal protection is required.
Why do both devices operate on one fault?
Because both see the same fault current, and the upstream one may be faster than the downstream one. Discrimination therefore needs two things at once: a sufficiently higher rated residual current and a time delay. Without either, half the installation goes down.
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.
More tools
- 01Cable cross-section
- 02Voltage drop
- 03Cable resistance
- 04Cable losses
- 05Maximum cable length
- 06Current-carrying capacity
- 07Parallel cables
- 08Conductor temperature
- 09Cable capacitance
- 10Short-circuit current
- 11Cable impedance
- 12Network impedance
- 13Short-circuit current at the far end
- 14Thermal short-circuit withstand
- 15Length and disconnection
- 16Motor current
- 17Torque
- 18Starting current
- 19Star-delta starting
- 20Soft starting
- 21Motor efficiency
- 22Speed control instead of throttling
- 23Speed and slip
- 24Setting the motor protection
- 25Single-phase motor
- 26Motor feeder
- 27Required motor rating
- 28PFD and SIL
- 29PFH and SIL
- 30Protective conductor size
- 31Earth rod
- 32Touch voltage
- 34Check discrimination
- 35Connecting a surge arrester
- 36Enclosure cooling
- 37Reference designation
- 38Reactive power compensation
Contact
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