Transcript
Introduction to High Impedance Faults
Hello, everyone, and welcome to today’s white paper webinar. Today we’re talking about high impedance faults. This is really a safety issue, and this affects utility customers as much or more than utilities. Utilities can have the same issue on the distribution network, but here we’re going to focus more on the customer side of the meter.
What we have for the high impedance fault is some sort of fault current flowing that is not high enough to trip any sort of overcurrent protection. Normally you have stuff like a circuit breaker or a fuse that is set to a certain high level of current. That current level is set higher than the normal load current, and the conductors all the way through that path are sized so it can safely carry the current up to the interrupt level of that protective device.
How Overcurrent Protection Works
For example, if you have a 20 amp circuit breaker in a residential situation, you have 12 gauge wire that is rated for 20 amps continuously, and if a fault occurs where you may have hundreds or even a thousand amps flowing through that wire, the circuit breaker will operate, preventing the connector from overheating. So every conductor in the path is rated for the current up to the trip point of that protective device.
As you go upstream, of course, the trip points are higher, the conductors are larger. And so you have a protection system where the further you are downstream, the lower that trip point. So you have protection coordination, as they call it. But all that assumes that a fault is a low impedance fault, that a short circuit is essentially zero ohms or close to it relative to the size of the conductor, which means that the current is high enough to actually make that protective device open up and stop the current flow.
What Makes High Impedance Faults Different
That works for low impedance faults where you have two conductors that touch or some sort of true short. But if you have something that’s a high impedance, that’s a different story. With a high impedance fault, you have a high resistance that is allowing a conduction path that shouldn’t be there, and that high resistance doesn’t produce trouble in terms of heat in the overall conduction path. The only problem is the heat right at that high impedance point.
For example, if you have two conductors that are barely touching through one tiny strand of wire or some sort of failure in insulation where you just have a really high resistance but not infinite connection there, you’ll have current flow, but the current flow will be lower than typical loads.
The Vacuum Cleaner Example
Here in the example in the paper, we have an example worked out for say a vacuum cleaner. It draws a little bit under 800 watts. Well, you can have a high impedance fault that draws less current than the vacuum cleaner. And so the conductors carry that current just fine. The protective device is not going to open because it doesn’t sense an overcurrent, because there really isn’t an overcurrent.
But all that power is located at that high impedance point. For the 780 watts in the vacuum cleaner, that power is being dissipated in a motor and wire that’s designed for that sort of power dissipation. Nothing’s going to get hot to the point where it fails. But if you have a high impedance fault, you may have even just a few watts or 10 or 20, 30, 40 watts here in this example. All that power is concentrated in a small space.
Localized Heating and Fire Risk
For example, here we have this extension cord, and there’s a poor connection there in one of the prongs of this cord. And you have very localized heating at that one spot. That localized heating is a hotspot that will exceed the temperature rating of the insulation, can start fires, and burn a home to the ground possibly, all while staying under the protection limit of whatever circuit breaker or fuse is protecting the circuit.
That’s the danger of a high impedance fault. You have localized heating that is enough to cause a fire or cause insulation failure, which then leads to a more catastrophic failure. A simple overcurrent protective device is not going to catch it because the current from this bad situation is lower than the actual load current, and certainly lower than the current carrying ability of the conductors in the circuit.
Detection Using Arc Fault Breakers and GFCIs
Generally, the protection around sensing that sort of current flow pattern — high impedance faults, especially the ones that have arcing components — sometimes have a high frequency signature, and that can be detected by an arc fault breaker or a GFCI in some cases, depending on the current flow. So protection schemes rely on not the magnitude of the current, but on the waveform of the current and trying to separate a normal waveform from a load from some sort of arcing waveform.
Arc fault breakers try to do that. They’re sometimes successful, sometimes they’re not. Sometimes they’ll have false positives or miss some, but they’re not perfect. But it is another layer of protection. So a breaker like that will have two different ways of operating. One is looking for that high impedance current signature, and the other is just pure overcurrent protection.
High Impedance Faults on the Utility Side
On the utility side, you have this sort of thing when you have arcing. You have an insulation failure. You have arcing from one medium voltage or even a high voltage connector to another. That arc current can flow for quite a bit of time and cause localized heating, but that current could be well under a breaker or recloser or other sort of protective device. And that can persist for many minutes or even hours.
At a substation or on an insulator on a distribution circuit, that arc current can continue to flow, and that’s a problem even though the current is not high enough to be a true fault current. That’s another source of high impedance fault. Those are difficult to detect on the utility side.
There are certain signatures like a high variability in the THD. Some substation relays can be programmed to try to trip on those sort of high impedance arcing signatures. But in general, that’s tricky to do because a false breaker operation at a substation is a pretty serious event in itself because you’re giving an outage to an entire region rather than just one circuit breaker in a home.
Summary and Contact Information
That’s what makes these sorts of problems dangerous. They can cause fires. They can cause failures from a simple 15 amp residential circuit in a piece of equipment like an extension cord, all the way up to distribution. And because there’s such a high variability in the level of current and the signatures involved, the waveforms, it’s very difficult to reliably have some sort of protective device. It’s not sufficient to rely on just simple overcurrent magnitude. These are a persistent problem. There’s no good solution in general. Arc fault breakers offer some protection, but at the cost of some false trips.
If you have questions, give us a call anytime at 1-800-296-4120 or send an email to support@powermonitors.com. Everyone have a great day, and thanks for attending.
