Tips to IT specialists on server room grounding
My name is Andrei Grebenik, and I am an electrical engineer. In my career, I have designed and built power supply grids for Kolenergo and RTRS, built and launched Beeline's base stations, designed and built power supply and grounding systems for a Rostelecom telephone exchange. I would like to share with you the purpose of the grounding system, how to deploy it, typical errors in doing so and potential consequences.
Why do you need correct grounding in your server room?
The design of grounding in the server room affects safety of equipment and human life. Properly arranged grounding helps:
- Avoid distortion of information during transmission over the network;
- Protect equipment during a thunderstorm;
- Save human life and health in case of equipment damage.
Design, construction and operation of grounding devices is a highly specialized area, and IT specialists do not need to understand it in detail. However, it is useful to understand the main principles: this will allow to operate the equipment without involving electricians for each task.
What is grounding?
In physics, grounding means establishing contact between something and the ground. Which way is not so important. The ground is like a bottomless barrel for electric voltage. At the grounding point, we obtain an electrical potential equal to zero. This is what the principles of grounding for electrical safety and interference protection are based on.
There is a problem — the ground is a poor conductor of electrical current, so we have to either create multiple points of contact or create a large contact surface.
The main characteristic of a grounding device is resistance to current flow. For IT equipment grounding, you usually need resistance of not more than 4 ohms and sometimes even lower. Equipment manufacturers often put this requirement forward.
The part in contact with the ground is called the ground lead. Most often it is provided by hammering or burying a metal element in the ground. Nevertheless, you can also use other options, such as a steel water pipe laid in the ground.
Once I participated in the construction of a grounding device for a server room at a construction company. The room stood on a 1-1.5 meter thick layer of filled-in ground, with solid rock beneath it. We drove ten rods into the ground and obtained the resistance of 100 ohms - 25 times greater than required. We were saved by finding a water well on the site and being able to connect to a pipe located in it. Eventually, we obtained the resistance with a two-fold safety margin.
A lesson learned here is if you need grounding, you do not necessarily need to dig the soil or drive rods. Look around; maybe there is already something to connect to. A rebar concrete foundation, a metal building framework will do, but not the heating or gas supply lines.
IT specialists do not usually build grounding systems by themselves. However, if you get involved in the process as a customer, here are three principles to help you:
- The thicker the ground leads material, the longer they will function. They are usually made of carbon steel L-beams and round bars, while pipes are used less often. The minimum standard thickness of the L-beam wing is 4 mm, the diameter of the vertical electrode is 16 mm, and the diameter of the horizontal ground lead is 10 mm. If you save on the thickness of the metal, then because of rust, the grounding will become useless more quickly. Therefore, the material used by designers and the construction company must be checked.
- It is better to have one 10 meters electrode than two 5 meters electrodes. If the soil allows, drive the metal part as deep as possible. The earth gets wetter with depth, and, respectively, conducts electric current better. Therefore, one long electrode will deliver better results than two electrodes of half the length. In the rocky ground, of course, it will be impossible, but in the sandy ground — quite so. About three years ago, I was building grounding devices in a residential home complex. There my fitting crew set a kind of record — they managed to drive the steel part to the depth of 15 meters.
- The distance between adjacent leads should be at least double their length. The ground leads located next to each other overlap the area of action and together produce a worse result than if distanced apart. Driving the rods in next to each other is a waste of effort and money.
In low voltage networks, the main type of protection against electric shock is disconnecting the power supply. Grounding itself is not used for protection, only as an additional measure. Nevertheless, it is useful because it reduces the electrical shock voltage.
The main problem is old grids, where grounding is only provided on the transformer and the building does not have grounding. On the way from the transformer substation the neutral lead gains potential, so the metal cases of the devices may start to "pinch". This can be a difficult problem to solve. Let us look at the options.
The minimum solution is provide the measure called "equalization of potentials". This is when the neutral lead at the entrance to the building is connected to all metal structures that can be reached - all metal pipes that enter the building, foundation reinforcement bars, air ducts. It would be perfect to provide “re-grounding” by connecting a ground lead at this point, but this depends on the budget available.
It is great when before installing the new equipment, the room is already prepared, and power supply and grounding lines are already is place. It is just as good to be able to do everything ourselves — we perform all the necessary works while observing the regulations. It is far more difficult when you have to deal with what you have, but the existing lines and equipment do not always meet the safety requirements. In this case, there are two options:
- If a five-lead cable (three-phase connection) or a three-lead cable (one-phase connection) is installed, we use the cable lead for grounding.
- If the cable is four-lead or two-lead, the risks are too high and we will rather leave the project. In case of damage to the neutral lead, the equipment enclosures will be exposed to voltage.
Interference in IT equipment is caused by many sources: short-circuits in power supply lines, lightning strikes nearby, or high-power equipment operating in proximity. Electromagnetic waves of noise signal overlap the useful data signal causing errors in data transmission and equipment malfunctions.
The main method of protection against interference where grounding is used is shielding cables and equipment with metal shells. The cable is shielded with a metal shell, while equipment is shielded by its metal enclosures. Then the shields are grounded.
The shield grounding method depends on the network throughput capacity. At the speed up to 10 Mbps the shield is grounded at one point, while at higher speeds — at several points.
Another issue that is often overlooked is the interference in the grounding system conductors themselves. This can be seen in systems where closed loop ground conductors are used. In such loops, the external electromagnetic field induces voltage, which creates interference. Two recommendations follow from this observation:
- Grounding should be arranged once from a single point. This way, no closed loops will be created in ground conducts.
- Ground conductors length needs to be kept to the minimum. The longer the conductor, the stronger the induced interference in it.
Short-term overvoltage (surge) occurs when switching electrical equipment on and off during lightning strikes. Powerful electromagnetic field is distributed over power supply wires, copper data transmission lines and even grounding conductors. Consequently, excessive voltage is induced in the equipment and it breaks down.
Here are a few protection methods:
- Using surge protectors (short-term overvoltage protection devices).
- Using galvanic isolation.
Surge protectors are gas discharge or solid-state devices, which are connected to the protected equipment on one side and to the ground on the other side. In case of overvoltage, the surge protector trips and drains the current to the ground. Here it is especially important to strictly follow the rules of equipment connection and the manufacturer's recommendations. Incorrect use will not only fail to protect the equipment, but also pose a threat to the equipment connected to the same grid.
Therefore, if nothing can be done according to the rules or codes, it is better to use alternative types of protection, for example — galvanic isolation of circuits. For communication lines, it means using optic fiber cables instead of copper cables — optic fiber does not conduct electricity, and therefore protects from overvoltage in communication lines.
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