Commissioning best practices: Electrical -- Top 10 deficiencies and challenges

These are the most common deficiencies or challenges workers face when working on a commissioning project.

10/17/2012


Here are the top 10 deficiencies or challenges encountered on a wide range of projects.

1. Equipment settings not available or not understood

Settings for ATSs and other equipment generally are not provided by the design engineer or owner prior to functional performance testing. The commissioning professional needs to request the settings well in advance of testing in order to allow time for the responsible party to calculate them. The commissioning professional should verify that the settings are correctly applied in the field.

2. Confusion regarding generator neutral bonding

Neutral bonding at the generator generally is not detailed on the design drawings and often is incorrect. Neutral bonding can be a complicated issue, but, in general, if 4-pole ATSs are used, the neutral should be bonded at the generator or paralleling switchgear. If 3-pole ATSs are used, the neutral should not be bonded at the generator or paralleling switchgear. In either case the neutral is bonded at the service entrance.

3. Incomplete sequence of operation

Sophisticated essential power systems often use paralleling switchgear that provides numerous load management features (load shed, bus optimization, etc.). Manufacturers often provide a standard sequence of operation that does not address customized system features. The commissioning professional can provide value by coordinating meetings with the design team, owner, and equipment vendors to discuss the features of the system in order to customize the sequence of operation. The commissioning professional will use the sequence of operation to prepare a detailed functional performance test procedure. This coordination meeting is an excellent time to review the ATS and paralleling switchgear settings as these settings are integral to the overall essential power system sequence of operation.

4. Missing or conflicting equipment labeling

Permanent equipment labeling is not applied until the majority of the electrical work has been completed, and this is after the majority of the electrical testing has been completed. Complete and accurate equipment labeling is very important for the operations and maintenance staff. The equipment labels listed on the drawings and sequences of operation should match the field labels and should be verified as part of the commissioning process.

5. Overcurrent device settings errors

The settings listed in the device overcurrent/coordination study do not match the available settings on the overcurrent device or are improper for the application<s>.</s>

6. Generator ground fault protection is provided on essential system equipment

The NEC does not require ground fault protection for the essential power system equipment. If the generator overcurrent device is incorrectly equipped with ground fault trip protection, often the ground fault feature cannot be disabled in the field. In these cases, the overcurrent device needs to be replaced. Ground fault alarming should be considered in lieu of ground fault trip protection for essential power systems.

7. Difficulty coordinating testing

Much of the incoming power and distribution equipment and some of the essential power system equipment are energized early in the project for use during construction. The project team may have difficulty remembering that testing of this equipment should occur prior to energization. Once the electrical equipment has been energized for use on construction projects, project schedules can make it very difficult to test, and shutdowns and after-hours testing will be required. This can drive up the project costs. An electrical test plan for each testing session can help communicate the testing schedule and scope to project team members and allow the contractors to incorporate critical testing dates into the master construction schedule.

8. Interface to mechanical systems not clearly defined

Sophisticated essential power systems often interface with mechanical systems such as fuel oil systems and generator room ventilation systems. The commissioning professional may need to request additional information prior to writing detailed functional test procedures. If any support system fails or is improperly wired, such as fuel transfer, supply or return pumps, the entire redundant generation system could fail. Therefore, commissioning all dependent systems under all anticipated conditions is required.

9. Resistive or reactive load banks

The use of a resistive versus reactive load bank for generator full load testing is commonly debated. The resistive load bank provides full kW loading to the generator and will adequately evaluate the engine cooling, exhaust, and fuel systems. In the case of more complex parallel generator systems, the reactive load bank may be desired to calibrate and test load sharing and voltage regulation of those systems. In most cases, resistive load bank testing will be sufficient for functional testing. However, UPS systems, other proportionately high total harmonic distortion (THD), leading power factor loads, high inrush, or other large problematic loads may all require special consideration when drafting commissioning procedures.

10. Using building load during generator testing

During generator testing load can be applied to the generator using load banks and building load. Connecting both building load and load banks often is not practical since most load bank testing occurs before adequate and consistent building load is available.


John Hebert is senior electrical engineer and has 15 years of engineering experience, including project management and technical experience commissioning electrical systems including medium- and low-voltage main power, emergency power, mission critical power, voice/data, fire alarm, and building automation and control systems. Catherine Melander is senior electrical engineer and has 30 years of engineering and project management experience. She has spent the past 12 years commissioning electrical systems, including incoming medium- and low-voltage power, emergency power, mission critical power, low-power, and building-wide “loss of power response” testing. 



No comments