Overcurrent Protection of Conductors
Before the conductor reaches the maximum allowable temperature, any short-circuit current passing through the cable must be interrupted by protective devices placed on live conductors. All conductors must be protected against overcurrent (refer to Clause 7.2).
Note: For neutral conductors, refer to the second paragraph of Clause 7.2.3.
Table D.5 – Maximum Allowable Conductor Temperatures under Normal and Short-Circuit Conditions| Insulation Type | Maximum Temperature under Normal Conditions (°C) | Maximum Short-Circuit Conductor Temperature (°C) |
| Polyvinyl Chloride (PVC) | 70 | 160 |
| Rubber | 90 | 200 |
| Cross-Linked Polyethylene (XLPE) | 90 | 250 |
| Ethylene Propylene Rubber (EPR) | 90 | 250 |
| Silicone Rubber (SIR) | 180 | 350 |
Note: - Conductors with tinned or bare copper are not suitable for prolonged short-circuit conditions above 200°C.
- For applications exceeding 200°C, silver- or nickel-plated copper conductors should be used.
- These values are based on an assumed adiabatic behavior where the duration does not exceed 5 seconds.
Calculation of Short-Circuit DurationIf the protective device is applied such that the total interruption time does not exceed 5 seconds (
t < 5 s), the short-circuit duration (
t in seconds) is calculated using the following formula:
t = ( k × S )² / I²
Where:- S = Cross-sectional area (mm²)
- I = Effective short-circuit current (A)
- k = Factor for insulated copper conductors based on insulation type:
- PVC: 115
- Rubber: 143
- SIR: 132
- XLPE: 143
- EPR: 143
Application of Protection DevicesgG and gM fuses (refer to IEC 60269-1) and circuit breakers with B and C characteristics (refer to IEC 60898) should be used to ensure that the temperature limits in Table D.5 are not exceeded. These devices must satisfy:
Iₙ ≤ I₂
as required in Table 6.
This structured approach ensures proper overcurrent protection for electrical conductors. Let me know if you need further modifications!
Understanding gG and gM Fuses
In electrical protection,
gG and gM fuses are types of
low-voltage fuses defined under
IEC 60269-1. These fuses are designed to protect electrical circuits from
overcurrents and short circuits.
1. gG Fuses (General Purpose Fuses)- Definition:
- The gG fuse is a general-purpose fuse designed for full-range overcurrent and short-circuit protection of cables and equipment.
- Applications:
- Protection of wiring systems, transformers, and motors.
- Used in distribution panels and household installations.
- Characteristics:
- Provides protection from both overloads and short circuits.
- Fast response to short circuits but allows a small delay for overloads.
- Commonly rated for AC and DC circuits.
✅
Example: A
gG 10A fuse can safely interrupt overcurrent
above 10A, protecting the circuit from damage.
2. gM Fuses (Motor Protection Fuses)- Definition:
- The gM fuse is a motor protection fuse, designed to handle the high inrush current (starting current) of motors while still providing short-circuit protection.
- Applications:
- Protection of electric motors and transformers.
- Used in industrial automation and control systems.
- Characteristics:
- Delays tripping during motor startup (which can have high inrush currents).
- Protects motors against prolonged overcurrents and short circuits.
- Usually combined with thermal relays for better protection.
✅
Example: A
gM 20A fuse allows
temporary overcurrents during motor startup, but will trip if the overcurrent persists.
gG vs. gM Fuses: Key Differences| Feature | gG Fuse (General Purpose) | gM Fuse (Motor Protection) |
| Protection Type | Overload + Short Circuit | Short Circuit Only |
| Delay for Overload | Moderate Delay | Higher Delay (Motor Start) |
| Primary Use | Cables, Transformers, General Equipment | Motors, Inductive Loads |
| Tripping Sensitivity | Faster Tripping | Slower for Overload, Fast for Short Circuit |
Which Fuse to Use?- If you need general electrical protection → Use gG fuses.
- If you need motor protection with inrush tolerance → Use gM fuses.