Protection Coordination & Discrimination
Overcurrent protection coordination — discrimination, back-up protection, and cascading
Disconnection Times
BS 7671 requires fault disconnection within specified times to limit the duration of touch voltage during an earth fault.
Maximum disconnection times for TN systems
BS 7671 Table 41.1| Circuit Type | Uo (V) | Max Time (s) | Regulation |
|---|---|---|---|
| Final circuit (socket outlets) | 230 | 0.4 | 411.3.2.2 |
| Final circuit (fixed equipment) | 230 | 0.4 | 411.3.2.2 |
| Distribution circuit | 230 | 5 | 411.3.2.3 |
| Special locations (e.g. bathroom) | 230 | 0.4 | 701.411.3.2 |
For TT systems, the same disconnection times apply but are typically achieved by RCD operation.
Verifying Disconnection
Time-Current Characteristics
Each protective device type has a distinct operating curve that determines how quickly it trips at different fault currents.
MCB instantaneous trip ranges
BS EN 60898| MCB Type | Magnetic Trip Range | Typical Use |
|---|---|---|
| Type B | 3-5 x In | Domestic, resistive loads |
| Type C | 5-10 x In | Small motors, fluorescent lighting |
| Type D | 10-20 x In | Large motors, transformers, X-ray |
In = rated current of the MCB.
BS 88 fuses have inverse time-current characteristics — higher fault currents result in faster clearance. Unlike MCBs, fuses do not have a fixed magnetic trip threshold, making them inherently better at providing discrimination when devices are rated in a 2:1 ratio.
Discrimination Ratios
Typical discrimination ratios for full selectivity
Manufacturer discrimination tables| Upstream Device | Downstream Device | Ratio Required |
|---|---|---|
| BS 88 Fuse | BS 88 Fuse | 2:1 |
| BS 88 Fuse | MCB Type B | ~1.6:1 |
| MCB Type B | MCB Type B | 2:1 (limited range) |
| MCB Type C | MCB Type B | Check curves |
| MCCB | MCB | Manufacturer data |
Full discrimination up to maximum fault level is only guaranteed when confirmed by the manufacturer.
Use Fuses for Critical Discrimination
Back-Up Protection (Cascading)
Back-up protection allows a device with a lower breaking capacity to be installed where the prospective fault current (Ipf) exceeds its rated breaking capacity, provided a suitable upstream device limits the energy let-through. This must be verified using the manufacturer's published cascading tables.
I²t (let-through) of upstream <= I²t (withstand) of downstream- I²t
- = Energy let-through in A²s during the fault
BS 7671 Regulation 434.5.1
Cascading Is Manufacturer-Specific