Transformers are essential components in electrical power systems, responsible for voltage conversion and power distribution. To ensure safety and reliability, the Philippine Electrical Code (PEC) provides specific guidelines for overcurrent protection of transformers. These provisions help prevent damage caused by excessive current, which may result from faults, overloads, or short circuits.
Overcurrent Protection Requirements
PEC Article 4.50 outlines the general requirements for transformer overcurrent protection. The level of protection depends on the transformer’s voltage rating, current capacity, and installation environment.
- Primary Overcurrent Protection (PEC 4.50.1.3(B))
- The primary protection device must be sized to prevent excessive heating and damage due to overcurrent conditions.
- For transformers with primary voltages of 1000V or less, the maximum rating of the overcurrent protection device (OCPD) is based on a percentage of the transformer's full-load current (FLC), typically:
- 125% of the primary FLC for most applications.
- If the standard OCPD size does not correspond to the calculated value, the next higher standard size is permitted, as per NEC 240.6.
- If secondary overcurrent protection is provided, primary protection may be increased to 250% of FLC.
- Secondary Overcurrent Protection (PEC 4.50.1.3(B) & 2.40.1(F))
- Overcurrent protection on the secondary side is required unless primary protection alone is sufficient to protect the transformer and secondary conductors.
- If primary protection does not exceed 125% of the primary FLC, secondary protection is not required.
- If secondary protection is used, its rating is typically limited to 125% of the secondary FLC, unless the next standard size OCPD is required.
- The NEC allows secondary protection up to 250% of the FLC for transformers supplying motor circuits.
- Overcurrent Protection for Medium-Voltage Transformers (Above 1000V, PEC 4.50.1.3(A))
- The NEC provides separate rules for transformers operating above 1000V.
- Primary overcurrent protection must be sized at 300% of FLC for non-fused circuit breakers or 250% for fuses.
- Secondary protection is required unless the primary device provides adequate protection per NEC requirements.
See also Harmonics Limit - What is the difference between the IEC and IEEE?
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Transformer Protection Considerations
- Current-Limiting Devices: To prevent extensive damage in case of faults, fuses or circuit breakers with current-limiting capabilities are recommended.
- Coordination with Downstream Protection: The transformer’s overcurrent protection should be coordinated with downstream devices to ensure selective tripping and avoid unnecessary outages.
- Protection of Transformer Conductors: PEC 240.21(C) outlines requirements for protecting secondary conductors, particularly for taps, which have specific length and OCPD location requirements.
- Special Applications: Some industrial applications, such as transformers used in motor circuits, may have unique overcurrent protection criteria as specified in PEC 4.30.6.2.
| Location Limitations | Transformer Rated Impedance | Primary Protection over 1000 Volts | Secondary Protection (Note 2.) | |||
|---|---|---|---|---|---|---|
| Over 1000 Volts | 1000 Volts or less | |||||
| Circuit Breaker See Note4.) |
Fuse Rating | Circuit Breaker See Note4.) |
Fuse Rating | Circuit Breaker or Fuse Rating |
||
| Any location | Not more than 6% | 600% (See Note 1.) |
300% (See Note 1.) |
300% (See Note 1.) |
250% (See Note 1.) |
125% (See Note 1.) |
| More than 6% and not more than 10% |
400% (See Note 1.) |
300% (See Note 1.) |
250% (See Note 1.) |
225% (See Note 1.) |
125% (See Note 1.) |
|
| Supervised locations only (See Note 3.) | Any | 300% (See Note 1.) |
250% (See Note 1.) |
Not required | Not required | Not required |
| Not more than 6% | 600% | 300% | 300% (See Note 5.) |
250% (See Note 5.) |
250% (See Note 5.) |
|
| More than 6% and not more than 10% | 400% | 300% | 250% (See Note 5.) |
225% (See Note 5.) |
250% (See Note 5.) |
|
|
Notes: 1. Where me required fuse rating or circuit breaker setting does not correspond to a standard rating or setting, the higher rating or setting that does not exceed the following shall be permitted: a. The next higher standard rating or setting for fuse and circuit breakers 1000 voltas and below, or b.The next higher commercially available rating or setting for fuses and circuit breakers above 1000 Volts. 2. Where the secondary overcurrent is required, the secondary overcurrent device shall be permitted to consist of not more that six circuit breakers or six sets of fuses grouped in one location. Where multiple oercurrent devices are utilized, the total of all device ratings shall not exceeed the allowed value of a single overcurrent device. If both circuit breakers and fused are used as the overcurrent device, the total of the device ratins shall not exceed that allowed for fuses. 3. A supervised location is a location where the condition of maintenance and supervision ensure that only qulaified persons monitor and service the transformer installation. 4. Electronically actuated fuses that may be set to open at a specific current shall be set in accordance with settings for circuit breakers. 5. A transformer equipped with a coordinated thermal overload protection by the manufacturer shall be permitted to have separate secondary protection omitted. |
||||||
| Protection Method | Primary Protection | Secondary Protection (See Note 2.) | |||
|---|---|---|---|---|---|
| Currents of 9 Amperes or More | Currents Less Than 9 Amperes | Currents Less Than 2 Amperes | Currents of 9 Amperes or More | Currents Less Than 9 Amperes | |
| Primary potection only | 125% (See Note 1.) |
167% (See Note 1.) |
300% (See Note 1.) |
Not required | Not required |
| Primary and secondary protection | 250% (See Note 3.) |
250% (See Note 3.) |
250% (See Note 3.) |
125% (See Note 3.) |
167% |
Notes: 1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted. 2. Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device. 3. A transformer equipped with coordinated thermal overload protection by the manufacturer and arranged to intem1pt the primary current shall be pem1itted to have primary overcurrent protection rated or set at a current value that is not more than six times the rated current of the transformer for transformers having not more than 6 percent impedance and not more than four times the rated current of the transformer for transformers having more than 6 percent but not more than 10 percent impedance. |
See also Engineers Need Actual Site Experience to Do Effective Design
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Conclusion
Proper overcurrent protection of transformers ensures compliance with PEC requirements while enhancing system safety and reliability. By appropriately sizing overcurrent protection devices on both the primary and secondary sides, the risk of transformer damage due to excessive current can be minimized. Engineers and electricians must carefully evaluate the transformer's ratings, installation environment, and PEC provisions when selecting and installing protective devices.