1 / 24

Chapter 6

Overcurrent Protection (Note: All the mentioned tables in this course refer to, unless otherwise specified, Low Voltage Electrical Installation Handbook, by Johnny C.F. Wong, Edition 2004). Chapter 6. General. Purpose Safety of Personnel (Shock) and Property (Fire Hazards)

lesa
Download Presentation

Chapter 6

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Overcurrent Protection(Note: All the mentioned tables in this course refer to, unless otherwise specified, Low Voltage Electrical Installation Handbook, by Johnny C.F. Wong, Edition 2004) Chapter 6 Electrical Installation 2

  2. General • Purpose • Safety of Personnel (Shock) and Property (Fire Hazards) • Maintain reliable life of equipment and systems • Overcurrent • a current exceeding the rated value of a circuit or the current-carrying capacity of a conductor • Overload • Fault • Short-circuit fault • Earth fault • This part, we are concerned with the short-circuit fault only. Electrical Installation 2

  3. Devices for Overcurrent Protection • Examples are: • Fuses (HBC/HRC) • Miniature circuit breakers (MCBs) • Combined MCB and RCD (RCBOs) • Moulded case circuit breakers (MCCBs) • Air circuit breaker + IDMTL relay Electrical Installation 2

  4. Devices for Overcurrent Protection • Protection for the NEUTRAL conductor is NOT required for TT and TN systems • 100% Neutral should be used • Protection already provided by the live conductor protective device • Neutral link (not protective device) • If the neutral breaks, the live supply must break too • LOSS OF NEUTRAL must be avoided to eliminate the risk of raising the potential of the load star point to dangerous level Electrical Installation 2

  5. Protection against Overload • Main purpose is to avoid sustained temperature that causes deterioration of insulation • e.g. only a short duration of overload current is allowed to flow in a motor circuit - the starting duration should be short. Otherwise larger cables shall be installed Electrical Installation 2

  6. Selection of Overload Protective Device • design current Ib nominal current or rated current In lowest CCC, Iz Electrical Installation 2

  7. Position of Overload Protective Device • At the point where there is a reduction of Iz (CCC) such as • CSA of conductor is reduced • Worsening of environmental condition • Change of cable type or installation method • Overload protective device and fault current protective device may be the same device and may be 2 different devices Electrical Installation 2

  8. Overload Protection of Conductors in Parallel • The Iz in this case is the sum of Iz of the individual cables provided they are in accordance with the conditions for parallel running cables. • Standard ring final circuits are not in this context. Electrical Installation 2

  9. Omission of Overload Protective Device • Overload current is unlikely to flow • Refer to Fig. 6.5 for illustration Electrical Installation 2

  10. Omission of Overload Protective Device • Unexpected loss of supply is more dangerous than overloading of circuit • Refer to Fig. 6.6 for illustration Electrical Installation 2

  11. Omission of Overload Protective Device • CT secondary circuit should not be broken. If this is the case, dangerous high voltage will appear at the CT secondary side • Refer to Fig. 6.7 for illustration Electrical Installation 2

  12. Omission of Overload Protective Device • Protection is afforded by electricity supplier’s protective device (not normally accepted by power companies in Hong Kong) • Refer to Fig. 6.8 for illustration Electrical Installation 2

  13. Protection against Fault Current • Cause - Insulation failure, faulted switching operation and invariably associated with arcs • Effect - Thermal and mechanical stress produced in conductors, associated support and plant components • Fault current protection is to prevent this Electrical Installation 2

  14. Protection for Maximum prospective fault current, Isc • Maximum prospective fault current, Isc • 3-phase : calculation based on symmetrical fault impedance, Isc = Up / Z where Up = phase voltage Z = phase conductor impedance at supply source • 1-phase : calculation based on line-neutral impedance at 20oC, Isc = Up / (Z + Zn) where Zn = neutral conductor impedance at supply source • The above should base on fault appeared just after the protective device • Breaking capacity of fault current protective devices should exceed the max. prospective fault current, Isc Electrical Installation 2

  15. Minimum Prospective Fault Current, I • Minimum prospective fault current, I • Calculation bases on total phase-neutral impedance values, up to the remote end I = Up / (Z + Zn+ Z1 + Z2) where Z1 = phase conductor impedance at consumer side Z2 = neutral conductor impedance at consumer side • Significant in determining fault disconnection time, t Electrical Installation 2

  16. Protection for Minimum Prospective Short Circuit, I • Basic equation to satisfy • k2S2 > I2t • Where • k - a constant associated with the type of conductor + insulation • S - Cross-sectional Area (CSA) of conductor • I- minimum prospective fault current (fault occur at remote end) • t- disconnection time • I2t - let-through energy Electrical Installation 2

  17. Guidelines in fault current protection • Max. prospective 3-ph symmetrical short-circuit at the l.v. source of supply provided by the supply company is 40kA. • All fuses and MCCBs at source of energy must have breaking capacity > 40kA • Fault current protective devices with smaller breaking capacities are generally acceptable if they are backed up by fuses to BS88-2.1 or BS88-6 (Backup protection will be discussed later in Chapter 10) • The further away from the source of supply, the smaller the prospective short circuit current. Electrical Installation 2

  18. Fault Current Protection in General • Example: The following single phase circuit is protected by 63A BS88 fuse, the prospective short circuit current at the fuse is known to be 3 kA. A connected load, with circuit distance 87m from the fuse, is to be supplied by using 16mm2 1/C PVC copper cable. Please check whether the fuse can provide short circuit protection for the cable. Source Installation side Source voltage Up Z Z1 63A fuse 1.68 Ω / km Load Zn Z2 Electrical Installation 2

  19. Fault Current Protection in General • At fuse position, it is given that the 1-Фprospective short circuit current is 3 kA, i.e. Isc = Up / (Z + Zn) Z + Zn = 3000 / 220 = 0.073 Ω The total impedance from the fuse to the remote load end, Z1 + Z2 = 2 x 87m x 1.68 Ω/km = 0.292 Ω So, the minimum short circuit current at the load end, I = Up / (Z + Zn+ Z1 + Z2) = 220 / (0.073 + 0.292) = 603 A Electrical Installation 2

  20. Fault Current Protection in General • Whether k2S2 > I2t ?? From I-t characteristic of BS88 fuse, t = 0.18 s when I = 603 A PVC copper cable is used  k = 115 S = 16 mm2 k2S2 = 1152 x 162 = 3,385,600 A2S I2t = 6032 x 0.18 = 65,450 A2S k2S2 > I2t  O.K Electrical Installation 2

  21. Fault Current Protected by Overload Protective Device • The protective device is assumed to be adequate if it • satisfies conditions for overload protective device. That is, we sizes cable and protective device by using the principle Ib≤ In ≤ Iz ; and • Breaking capacity of protective device ≥ Maximum prospective fault current, Isc • This is the most common way to protect a circuit, since only ONE protective device is needed. Electrical Installation 2

  22. Position of fault current protective device • Normally placed at or before the point where a reduction in the conductor’s current-carrying capacity (Iz) occurs. Such change may be due to a change in: • cross-sectional area, method or installation, type of cable or conductor, or in environmental conditions Electrical Installation 2

  23. Fault current protection of conductors in parallel • A single device may provide protection against fault current for conductors in parallel provided the parallel conductors are in accordance with Section 5.8 Electrical Installation 2

  24. Omission of short-circuit protective devices • Conductor between a transformer and its control panel • Refer to Fig. 6.18 for detailed illustration Electrical Installation 2

More Related