Electro-Mechanical Protective Relay

The Electro Mechanical Protective Relay was once the backbone of higher voltage electrical power distribution systems.  Although digital relays are used today, there remains a high percentage of older equipment that still relies on these legacy devices.  Terms like amp tap, time dial, instantaneous, IAC, CO, IFC all are part of the legacy or electro-mechanical relays.  This video is based on material found in electrical power training classes at: Brainfiller Courses

Video Transcript

Electromechanical relays have been around for decades. They’ve been giving way to the digital, or the microprocessor-based relays, but there’s still a tremendous installed base of electromechanical relays. So if this is new to you and you’re at a facility or you have a client that has electromechanical relays, what I want to do is explain what some of the settings are all about.

Looking at this diagram right here, this is actually an electromechanical relay with the cover off of it. And you see that I have an arrow pointing to something known as the Amp Tap and another arrow pointed to basically like a thumb wheel that says Time Dial. And then over on the right side, it says Instantaneous. And what these settings are all about, these settings allow you to adjust how this protective device is going to operate.

So to begin with the Amp Tap– it’s actually a little horizontal bar across the top. And you could see there are numbers on it. And the numbers indicate 4, 6, 8, 10, 12, and 16. And what those are all about is there’s a plug, a screw-in type of a device, and you plug into one of those numbers. For example, maybe the 4, or the 6, the 8, depends on how you want this device to respond. And that defines the magnitude of current at which this relay will begin to operate or should begin to operate. I say should, because once you hit exactly that current, since it’s electromechanical, there may be a little bit of a dead band in there.

Now the Time Dial, what that is all about, that is to adjust how fast the device will operate, the timing, which is why it’s called a Time Dial. So if you actually look down towards the bottom of this relay, there is a circular disk down at the bottom. And so what happens when the current coming into this relay exceeds the Amp Tap setting, that disk begins to rotate. And there’s a vertical shaft connected to the disk.

And you could see another disk, a smaller disk, halfway between the Time Dial and the big disk on the bottom. And that smaller disk will also rotate. And to the right of the picture, you see there’s a small little rod, a vertical rod. And what’ll happen is that’s rotating around. And when it rotates around, it would rotate towards us, and then go over to the left.

And there’s a contact on the left. There’s a T there. And if you look to the right of the T, there’s a horizontal contact. And so that rod will hit the horizontal contact, and then that will initiate the breaker tripping. So it’s a very simple device.

And you can adjust the timing with that Time Dial. Right now the Time Dial is shown adjusted to a 5, and that’s where it defines where that rod is positioned. But if we were to make a setting adjustment to say a 6, or a 7, or an 8 Time Dial, then what’s going to happen is that that post or that rod’s going to be parked further away, and it’ll take longer for it to travel before it hits the contact and sends a trip signal to the breaker. So you’re adjusting the timing just by where the position is of that rod, you do that through the Time Dial.

And then on the right side, the Instantaneous, that’s really nothing more than an electromagnet. Up at the very top, you see a little indicator it says 80 and 60. And it actually goes down to 40 and lower and little bit higher. And what that indicates is the current that it would take for this device to operate instantaneously. It’s like an electromagnet.

And so there’s a hex bolt. You see it up at the top. And that bolt, the further that you crank it down into the coil, the less current it takes to trip instantaneously. The more that you back it out of the coil, the more current it takes to trip instantaneously. So it’s a very simple device.

The problem is for the Instantaneous on an electromechanical device, you can’t really visually set it. So what you need to do is perform testing, bench testing, to see exactly where this needs to be set to achieve your desired goals.

This relay– this is a different picture of the relay. Down at the bottom, there’s this black type of a bar that pulls out. You see there’s some metal fingers towards the back of it. That’s a shorting bar. Because what happens, you have the output of current transformers coming into this, and you can’t open circuit a current transform. It’ll flash. You’ll get a very high voltage. It’s a dangerous condition.

So this shorting bar, what it does when it’s inserted into the relay, it makes contact so that the relay is actually receiving it’s signal from the current transformers. And when you pull this out, there are fingers or contacts in the back that will actually close shut and short out the current transformers. And then when you short out the current transformers, you can actually pull the relay out of it’s enclosure.

This picture shows a close-up of the relay. And you can see I actually have the Amp Tap inserted into the number 4 position right here. So that indicates that when four amps or greater comes into this relay off of a current transformer, then it will begin to respond, or it should begin to respond. Again, there there’s a little bit of a dead band in there.

And the Time Dial is set right around the 5 Time Dial. Looks like the Instantaneous is set at about a 40. But, again, you actually have to test this to know exactly where it should respond.

You can take these relays and actually pull them out of their case. And to pull them out of their case– the shorting bar that I was talking about– you pull the shorting bar out, so that’ll short the current transformers. And once the current transformers are shorted out, then you can safely pull the device out without tripping the circuit. So you could do this without tripping a breaker. And you can perform testing or maintenance without taking a circuit offline.

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