Today I’m going to give you a brief overview of three different types of grounding systems that are important.
These three systems are:
- Ungrounded Systems
- Resistance Grounded Systems
- Solidly Grounded Systems
I’ve already talked a little bit about what grounding is, including giving a brief overview of why we do it and what it’s used for. If you haven’t read that article yet, go give it a read before continuing.
Finished reading about what grounding is? Alright then, let’s get to the meat and potatoes of today’s topic.
“Whoa, hold on”, you might be thinking, “We just finished reading about how important grounding is for safety! Why would we have ungrounded systems?” The answer is that we shouldn’t really have ungrounded systems, but they do exist and they do have their purposes.
You see, an ungrounded system isn’t really ungrounded. Electrically, your system is connected to ground through the capacitance between the lines and the earth, so you can say that it’s a capacitance grounded system. We just call it ungrounded because of convention, and because there isn’t a direct physical connection between any of your power lines and the ground.
There are a few advantages to having an ungrounded system. The first is that since your system is never physically connected to the ground, you have negligible ground fault current. For example, in a 3-phase system, because all of the ground fault current is capacitive, when you have a single line-to-ground fault in an ungrounded system, the current and voltage you would lose is negligible, and is instead carried by the other two lines. This allows you to continue operation unimpeded during a single line-to-ground fault.
The other big advantage is that because of the negligible ground fault current, special ungrounded systems can be used to minimize shock risk to people. An excellent example would be medical equipment in a hospital: a patient is directly connected to the machine, and if a fault were to occur, electricity might be able to flow through the patient and into the ground. Because the ground fault current is negligible in an ungrounded system, no power current will pass from the machine through the patient into the ground.
Of course, the disadvantages of an ungrounded system are obvious. If there is a fault, you are now using two wires to carry an amount of current that was allotted for three wires: the increase in current and voltage will jack up the heat, and the extra heat will wear out your insulation much faster. Worn out insulation could lead to unnecessary damage to your electrical system, particularly at motors.
The other big disadvantage of an ungrounded system is that it is incredibly difficult and time consuming to locate any faults. Each line must be tested individually, which is a very slow process that completely interrupts service. The opportunity cost of a fault in an ungrounded system is very high.
Ungrounded systems were the norm back in the 40’s and 50’s, but because their disadvantages outweigh their advantages in most scenarios, you won’t see too many new ungrounded systems today.
Resistance grounding is when you have a connection between your neutral line and the ground through a resistor. This resistor is used to limit the fault current through your neutral line: if your voltage doesn’t change, then your current is dependent on the size of the resistor according to Ohm’s law (V=IR).
Advantages Over Ungrounded Systems
Because the current in the neutral is controlled instead of negligible, system overvoltages are also controlled. This reduced current and reduced overvoltage means reduced heat, which keeps the wear and tear of your electrical system to a minimum. This is especially important for keeping your motors safe, since the reduced current will not damage the magnetic iron of the motor (which is costly to repair). The reduced currents also reduce the risk of shock and arc flash/blast hazards.
There are two types of resistance grounding: high resistance grounding and low resistance grounding.
High Resistance Grounding
High resistance grounding is typically used to limit ground fault current to < 10 amps. The low ground-fault current also means that, just like an ungrounded system, you can continue to operate the system on a single line-to-ground fault. The low current will typically not trip your protective devices during a single line-to-ground fault.
Overall, you want to use high resistance grounding when you need low fault current and still want to operate with a single fault. High resistance grounding is typically seen in retrofits of previously ungrounded systems in addition to new systems.
Low Resistance Grounding
Low resistance grounding typically limits ground fault current to between 100 and 1000 amps. This offers a similar advantage to high resistance grounding in that you can control the ground fault current, which means you can design your system to withstand the currents without damage.
Low resistance grounding systems have the benefit of tripping your protective devices when there is a fault. Their purpose is to immediately cut the power to the circuit, and so unlike the high resistance grounding systems, a low resistance grounding system will not maintain operation during a single line-to-ground fault.
Low resistance grounding also reduces overvoltage, and is used in medium voltage systems of 15kV or less, typically where big generators/motors are used.
Solid grounding is what you get when you connect your system directly to the ground, without any sort of resistance in the way. The ground is typically connected to the system at a neutral point, like the neutral terminal of a generator or transformer.
Pros and Cons
Solid grounding, like resistance grounding, can greatly reduce overvoltages in your electrical system. However, solidly grounded systems have the potential to have huge amounts of ground-fault current. As a result, solidly grounded systems cannot operate with a ground fault (since all of the current in the system is going from fault to ground).
Solid grounding has two main uses:
- In systems with voltages of 600V or less, solid grounding can be used if it is not necessary to maintain operation of a faulted circuit.
- In systems with voltages of 15kV or greater, solid grounding can be used if high ground fault currents are desirable of any reason, such as quick ground fault detection (since the high current will most definitely trip protective devices).
- You can use ungrounded systems when you want to have negligible ground-fault current.
- Resistance grounding offers the advantages of ungrounded systems without the risk of large overvoltages.
- Solid grounding reduces overvoltages but has high ground-fault currents.
At the end of the day, the type of grounding you use for your system will depend on which type of grounding best suits your needs and budget.