Medium-Voltage Cabling Quality

The rise in data center usage bolstered by the move to remote work and distance learning has put pressure on data center owners to deliver facilities faster while increasing capacity. Today’s mission critical facilities require increasing amounts of power, which lends itself to the use of medium voltage (MV) equipment and cabling in many data center designs. While MV can deliver more power in a smaller footprint than an equal capacity low voltage system; the MV installation comes with significantly more sensitive cable construction, installation practices, and complicated protection schemes. For these reasons, medium voltage cable and terminations must have significant focus on quality control. Done improperly, the lifetime of an MV cable installation can be drastically reduced from say 40 years to just a couple of years. These early failures will cost millions of dollars’ worth of rework after the building is occupied and can create an unsafe work environment—not to mention a loss in uptime to the facility. Fortunately, there are steps contractors can take to ensure a quality MV installation to extend the life of the system without
adding cost.

IMPORTANCE OF UPFRONT VERIFICATION
Upfront verification is crucial to ensuring the cable is designed properly for the installation. It is important to involve your local MV cable and MV termination kit vendors early in the process to review and confirm that the MV cable selected will meet the design intent. These vendors provide highly technical resources that are crucial in product selection and compatibility.

Additionally, a heat calculation and a pull calculation must be performed during the design phase to ensure the proper sizes of conductors and raceways. Heat calculations, also known as Neher McGrath calculations, are essential in underground installations to confirm the adequacy of heat dissipation of the conductors, thus confirming the ampacity size of the cable. Pull calculations confirm that the conduit and cable sizes will work together without damaging the MV cable due to excessive side-wall pressures nor exceeding cable tensions limits.

Once the MV cable design is confirmed and prior to buy out, a rigorous submittal process must be completed to ensure the proposed cable meets the design specifications. Look for:

1. (1) Manufacturer
2. (2) Voltage Rating
3. (3) Conductor Type (Copper or Aluminum)
4. (4) Insulation Thickness (100% or 133%)
5. (5) Insulation Type / Color
6. (6) Semi-conductive Layer (semicon) and Jacket type compatibility
7. (7) Temperature Rating
8. (8) Tape Shield % Overlap
9. (9) Termination kit compatibility

UNDERSTANDING THE INSTALLATION
Once the entire MV cable system has been confirmed, now we need to understand the installation. Because of the layers within and the materials used, MV cable is sensitive to bending, twisting and over-pulling. To account for this and properly plan the installation, teams should take the following steps:

• Re-review Pull Calcs – Double check the design pull calcs based on the physically installed layout.
• Maximum Bend Radius – (10x – 12x diameter) – refer to your specifications and manufacturer’s requirements. These radius requirements need to be met within the equipment as well. If
  you bend a cable past the requirements, it may need to be replaced entirely.
• Use of a tension meter – There are digital meters that can record the tension on the cable during installation. Keeping a record copy shows due diligence and removes the finger pointing if a cable fails in the future.
• Termination Training – All people terminating shall have physical training on the specific termination before the permanent install.
• Use a gauge for measuring cutbacks – if the installation has many terminations of the same type, a metal gauge is helpful to ensure a consistent process.
• Execute with a checklist!
• Ensure that an independent QA/QC process is in place
• After testing (see next column):
  • Torque verification – Make sure you torque, mark, and record your termination per specifications.
  • MV Boots or Fire Tape – Know the spec requirements beforehand, as boots have a lead time.

TRAINING
Training is probably the most important aspect of a quality medium-voltage cabling installation. The typical practice is for the electrical contractor to get a 1-hour Slideshow presentation from the selected termination kit vendor. After this presentation a certificate or card is given to the participants and they are considered certified. The problem is the training offers no hands-on experience, which oftentimes leads to incorrect terminations even after the training.

The best solution to this problem is to develop a sophisticated project specific training program and a termination checklist. The team comprised of the contractor, termination kit vendor, the QA/QC inspector, and the owner should develop the termination checklist before the training. It should incorporate all project requirements into a single checklist. This allows all parties to coordinate and make decisions on the best requirements to follow for the specific build.

Each project-specific training program should require workers to complete the developed termination checklist in a controlled learning environment using actual cables and term kits for hands-on experience. It is not recommended that this training be done on a permanent installation. Also, the QA/QC inspector for the terminations must approve a successful termination before the worker is considered certified.

Lastly, the certified workers should be logged on an approved list visible to the site team. Each final field termination should be done by a certified worker. It is important that each completed termination be tagged to identify the worker associated with the termination.

FINAL TEST SELECTION
Finally, testing is important to confirm there are no blatant defects in the cable from either manufacturing or installation. The testing available on the market today includes:

• DC HiPot – This has been known to be a destructive test, although this depends on the manufacturer and EPR type. The test is too sensitive on specific site conditions, thus the results
  are pass/fail.
• VLF Withstand – This is the most common test used today. As with the DC HiPot test, the results are in uA and is pass/fail.
• VLF Tan-Delta – A cable can basically be modeled as a resistor and capacitor in parallel. When a voltage is applied to the cable, the total current will be contributions of the resistive current and the capacitive current. Tan Delta is the ratio between these currents, which gives you a benchmark, not necessarily a pass/fail.
• Partial Discharge Test (PD) – Partial discharges are small electrical sparks that occur within the insulation of medium and high voltage electrical assets. Partial discharge testing results can help predict future performance and reliability of cables. This also gives you a benchmark, rather than pass/fail, to check for defects and assess options.

While each project team needs to determine what testing best fits their needs, a simple VLF withstand test is suitable for most needs in the industry.

A SEAMLESS INSTALLATION
The cable passed the selected test(s), so that means everything is good to go and will perform at its expected capacity and output, right? Not necessarily. A passing test does not always mean the cable is without defects. Smaller nail or pin hole penetrations into the insulation may not be picked up in the test at all. Therefore, the previous steps of verification and understanding the installation are important to use with the testing to certify a high-quality installation. Following those steps not only heads off potential issues with installation and the cable itself, but it also prolongs the life of the cable to deliver faster while maximizing client dollars.

Mike Fernholz is Engineering Services Manager at JE Dunn Construction. He can be reached at [email protected].