Q

Could an isolated ground cause PDU failures?

Isolated, dedicated grounds, telecom grounds -- the performance and, more importantly, safety of data center equipment -- rely on proper grounding.

I am confused by your article pertaining to isolated grounds. I have been working in data centers for over 20 years and the most common issue I encountered is that, when most of the equipment is utilizing AC power along with a high density of non-linear loads, there are always high currents on the grounding system. This causes AC system overheating, PDU failures, transformer failures and so on. The easiest way to eliminate these currents...

is to isolate the AC grounds from the equipment grounds. Most of the major Internet providers require this so [your article] goes against everything I have learned. Am I wrong in my understanding of this practice?

An isolated ground system, not dedicated grounds, is potentially more problematic than helpful in a data center -- there's a difference. If you're running an uninterruptible power supply with a 120/208 volt output with transformer isolation, then the UPS is classified as a separately derived source and a neutral/ground bond is required at the output.

Grounding requires bonding signal reference structures, like a raised floor grounding grid, to the equipment grounding conductor. There could be several power ground points in the electrical system of a large data center in addition to cabinet grounds, cable tray grounds, air conditioner grounds and others.

The actual ground for all of these has to come from code that is all the way back at the incoming building service ground (usually in the main electrical room in the basement). But that ground must also be tied to building steel, the incoming water pipe and perhaps several other things depending on the building and electrical system designs.

Let's assume the telecommunications grounding system is designed in accordance with the ANSI/TIA 607-B standard, as it should be. It must also tie from the main telecommunications ground bar (MTGB) back to the building entrance ground. All the rest of the telecom ground bars in the building connect back to the MTGB, but are also bonded to local electrical panels and building steel in each telecom room. The ground system in your data center is anything but isolated.

Contrary to being isolated, a good telecom ground is actually integrated into the building power ground. A true isolated ground requires complete separation from the telecommunications grounding system -- a bad and probably impossible idea since most data centers have critical telecom service terminations that connect to the telecom ground. The fully isolated ground would have to be separately connected all the way back to the building entrance ground with a large copper conductor. This is expensive as well as redundant. The isolated ground is probably ineffective since the data center and telecom grounds probably tie together.

But let's say you have a single UPS with 120/208 volt output. You bond the output per code but make the ground truly isolated by bringing the grounding conductor from the building entrance ground with its own large conductor. You require separate, isolated ground bars in each circuit breaker panel, each connected back to your UPS to form a single point ground. You independently run the grounds for raised floor, cabinets, cable trays and everything else back to that single ground point. This is a true isolated ground system within your data center.

The problem arises when you try to connect the cabinet power strips, which are often power distribution units (PDUs) that don't use isolated ground receptacles. The single ground wire you run to each power strip provides both a circuit ground and a safety ground for the metal case of the power strip. The metal power strip is mounted to the equipment cabinet, which is also tied into your isolated ground. Screws mount metal-cased equipment in the cabinet, which is also grounded with cords and plugs.

You've now introduced several additional -- yet unplanned -- pathways to the carefully isolated ground, forming ground loops that generate differences in ground voltage due to different path lengths. Voltage differences cause eddy currents, resulting in disturbances in the electronics equipment collectively called grounding problems. Insulated screws and washers keep chassis electrically isolated from the cabinets.

I suggest installing a robust grounding system that solidly ties everything together, also referred to as a brute force ground -- it forces everything in the room to be at the same electrical ground voltage (equal potential ground).

You may have this power design in the data center without realizing it. Hopefully, you haven't removed safety grounds to remove ground loops. But whatever you did got rid of stray paths that were causing eddy currents and created a system of solid grounds where everything is bonded together with copper conductors. You should also have green grounds from the cabinet power strips back to the circuit breaker panels. These may all terminate on ground bars in the breaker panels that are separate from the neutral bars, but they likely connect together inside the panel or at the PDU transformer, if it has one.

If the receptacles on cabinet power strips aren't orange, or don't have a little triangle symbol on them, then they are not isolated grounds.

Overheating transformers are likely not K-rated, which means they are not designed to handle the heat generated by harmonic loads. Harmonics are additive on the neutral conductor, and in a three-phase system, the maximum allowable flow is the current in one phase conductor multiplied by √3, or approximately 1.73. Since conductors aren't made in 173% increments, double the size of the neutral conductor in three-phase systems serving harmonic loads to use standard wire gauges.

It's unclear why revisions to the grounding system would solve this -- harmonic currents build up on the neutral conductor, not on the ground -- unless the ground is mis-wired and acting improperly (and illegally) as part of the neutral. Modern equipment should be harmonically neutralized, making oversized neutrals and K-rated transformers an unnecessary precaution.

For the data center as a whole, a robust, dedicated grounding system complying with the National Electric Code and ANSI/TIA 607-B should be sufficient.

This was first published in July 2014

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