A switch does not have a single breaking current. IEC 62271-103 defines a rated current for each operation: active load, closed loop, cable charging, line charging, earth fault, motor, capacitor banks and short-circuit making. The person specifying must ask for those the network needs.
Short answer
IEC 62271-103 (edition 1.0, 2011) does not assign a single breaking current to a switch. It defines a rated current for each type of operation. The manufacturer declares those the equipment has and the person specifying asks for those the network needs. All are chosen from the R10 series (1 – 1.25 – 1.6 – 2 – 2.5 – 3.15 – 4 – 5 – 6.3 – 8 and their multiples of ten).
The rated currents, one by one
| Symbol | Name | What it is | Clause |
|---|---|---|---|
| Iload | Mainly active load breaking | Maximum active load current broken at its rated voltage. It equals the rated normal current if the nameplate gives no other value | 4.101 |
| Iloop, Ipptr | Closed-loop breaking | Maximum closed-loop current broken. One value can be assigned for the distribution line loop and another for parallel transformers | 4.102 |
| Icc | Cable charging | Maximum cable-charging current broken | 4.103 |
| Ilc | Line charging | Maximum overhead line-charging current broken | 4.104 |
| Isb | Single capacitor bank | Maximum current broken with no adjacent bank on the source side | 4.105 |
| Ibb | Back-to-back capacitor banks | Maximum current broken with other banks on the source side | 4.106 |
| Iin | Making with inrush current | Peak value made with back-to-back banks | 4.107 |
| Ief1 | Earth fault | Maximum earth-fault current broken in the faulted phase, in networks with non-effectively earthed neutral | 4.108 |
| Ief2 | Cable and line charging under earth fault | Maximum current broken in the healthy phases, in the same type of network | 4.109 |
| Imot | Motor breaking | Maximum steady-state motor current broken | 4.110 |
| Ima | Short-circuit making | Maximum peak current made at its rated voltage | 4.111 |
How each one is understood
Mainly active load. This is the current of a circuit whose power factor is at least 0.75, where the load is represented with resistances and reactances in parallel (clause 3.7.102).
Closed loop. This is breaking in a circuit where both sides of the switch remain energized after opening: a distribution ring or a transformer in parallel with others (clause 3.7.104).
Cable and line charging. This is the capacitive current drawn by an unloaded cable or overhead line (clauses 3.7.105 and 3.7.106). Table 1 of the standard gives preferred values for both according to rated voltage.
Earth fault. In a network with non-effectively earthed neutral there are two different situations. Ief1 is the current the switch breaks in the faulted phase when clearing an earth fault of an unloaded cable or line on the load side (clause 3.7.111). Ief2 is the current it breaks in the healthy phases when disconnecting an unloaded cable or line while the earth fault is on the source side (clause 3.7.112). The standard notes that the maximum of each is 3 times the normal charging current, a case that occurs with individually screened cables.
Capacitor banks. There is one value for a single bank and another for back-to-back banks. In the second case, the standard requires the inrush current (Iin) to be assigned as well, whose frequency can be between 2 kHz and 30 kHz. The switch cannot necessarily break that inrush current.
Motor. The motor breaking current refers to IEC 62271-110. Unless otherwise stated, with the rotor locked 8 times the rated motor current is taken.
Short-circuit making. This is the maximum peak current the switch can make. In a general-purpose switch it equals the rated peak withstand current (clause 4.112).
What each switch type has
- General purpose: must have Iload, Iloop, Icc, Ilc and Ima. If it will be used in networks with non-effectively earthed neutral, also Ief1 and Ief2 (clause 4.112).
- Limited purpose: has the rated normal current, the rated short-time withstand current and one or several, but not all, of the capabilities of a general-purpose switch (clause 4.113).
- Special purpose: in addition to the above, it may have breaking capability for parallel transformers, single capacitor bank, back-to-back capacitor banks or motor (clause 4.114).
If the switch is backed up by fuses, the short-circuit and making currents can be chosen considering the current-limiting effect of the fuses. IEC 62271-105 is used for that (clause 4.115).
- Does your network have a non-effectively earthed neutral?
- Yes: Iload, Iloop, Icc, Ilc and Ima, plus Ief1 and Ief2
- No: Iload, Iloop, Icc, Ilc and Ima
For capacitor banks and motors, the standard provides the special-purpose switch, with Isb, Ibb and Iin, or with Imot.
What not to do
- Do not assume a switch breaks any current in the list. Request each value in writing and read the nameplate.
- Do not ask only for the rated normal current for a cable circuit: Icc is missing.
- Do not use a general-purpose switch for banks or motors without confirming the specific current Isb, Ibb or Imot.
- Do not confuse Ief1 with Ief2: they measure two different currents of the same fault.
Related topics
Closed-loop switching is typical of an open-ring medium-voltage network. Cable-charging breaking depends on the installed power cable. For banks, see the capacitor bank. The peak making current comes from the short-circuit study. Its base equipment is introduced in the guide on the medium-voltage switch-disconnector.
The electrical classes of the switch are explained in classes E1, E2 and E3 of the load-break switch. For capacitor banks and motors there is the special type: special-purpose switch.
The other parts of IEC 62271 are laid out in the standards for primary equipment testing.
When to call us
At Tiza Group we define with you the rated currents each switch in your network needs, from the short-circuit study through to the specification. See our primary engineering design service. Write to us and we will send you a quote.
Related guides
- Classes E1, E2 and E3 of a load-break switch
- Special-purpose switch
- Medium-voltage switch-disconnector and RMU and their tests
- Open-ring medium-voltage network: operation, faults and RMU
- Power cables: parts, types, protections and tests
References
- IEC 62271-103, High-voltage switchgear and controlgear - Part 103: Switches for rated voltages above 1 kV up to and including 52 kV, 2011 edition: clauses 3.7.101 to 3.7.118 and 4.101 to 4.116 (Table 1).
Frequently asked questions
What breaking current does a switch have if the nameplate gives no other value?
The mainly active load breaking current, Iload, equals the rated normal current if the nameplate gives no other value, according to clause 4.101 of IEC 62271-103.
Why is there one earth-fault current and another for charging under earth fault?
They correspond to two situations in a network with non-effectively earthed neutral. Ief1 is the current broken in the faulted phase. Ief2 is the cable or line charging current broken in the healthy phases when the fault is on the source side. The standard states that the maximum of each is 3 times the normal charging current.
Which breaking current is used for a motor?
The motor breaking current, Imot, is the steady-state current the switch can break. Unless otherwise stated, with the rotor locked the breaking current is 8 times the rated motor current. The standard refers to IEC 62271-110 for this case.