ASK THE EXPERTS: Hydro Grounding

Intermediate

I’ve installed several small microhydro systems without including grounding. It doesn’t seem necessary or safe to create a ground in small isolated systems that generate just one target voltage. I think it only creates an extra way to get shocked. Big systems may deliberately use the earth as a conductor and grounding protects people from high pre-transformer voltage shocks. Am I right in my antigrounding stance?

Rob Endicott • via email

If everything is properly installed and working correctly in an “ungrounded” system, it will work fine. But if something goes wrong, there is nothing there to protect the users and components. It’s like driving a car without bumpers and seat belts—as long as you don’t get in an accident, you are OK. Grounding is there for when things go wrong.

Grounding can be confusing, but it is important for all electrical systems—even in small, low-voltage hydro systems. While it is true that there is much less shock potential from a low-voltage system, that is only one reason why systems require grounding. Grounding reduces the hazard potential through several different means:

  • Connecting the electrical system to grounding electrodes (ground rod) bleeds away any stray voltages so they won’t damage sensitive electronics.
  • Connecting all of the metal enclosures and conduit in the system with an additional grounding conductor provides a safe path for electricity to flow when a fault occurs, to prevent shock and fires.
  • Connecting one of the electrical conductors (in DC systems, usually the negative conductor) to the ground system allows the breakers to trip or fuses to blow when a fault occurs. This protects the system and users, and makes it obvious if there is a problem with the system. This connection must be done at one point only in the system to avoid ground loops (unwanted current on the grounding system), which can be caused by parallel paths to ground.

The National Electrical Code (NEC) allows DC systems operating at 50 volts or less to be “ungrounded,” but this only means the DC negative conductor is not intentionally connected to the grounding system. A ground rod and ground conductors connecting all the metal enclosures and conduit are still required. In addition, since both the negative and positive conductors are now “ungrounded,” the NEC then requires overcurrent protection and disconnect devices in both the positive and negative conductors. This adds significant cost and complexity to a system, so grounding the DC negative conductor is usually preferred. 

In summary, a microhydro system without grounding can work, but a system with proper grounding will be safer and less likely to be damaged when a problem occurs. See my article on grounding in HP118 for more explanation and information.

Christopher Freitas • Sun Energy Power International

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