Relay upgrades can be complicated projects and getting started on the right foot is important.  Decisions made early in the process can create big problems downstream.  Like everything complicated, if you break it down into smaller pieces it gets a lot easier.  In this post we will take on one element of a relay upgrade project, specifying the right relay(s)

In most cases, the intention is to upgrade the existing protection system to newer relays without changing the protection scheme itself.  Even if you want to add functionality, such as thermal protection or automation you will start with these basics and add as you need.

The basic steps are as follows:

  • Determine what protection you currently have
  • Verify your control voltage
  • Determine your Current Transformer ratio and secondary output current.
  • Develop relay part number(s) to duplicate the existing protection

The first step can appear daunting, but you have all the information, you just need to know how to read it.  All electromechanical relays are single function, devices which means each relay performs one specific function. The goal is to replace these may single function devices with one or more multi-function devices.  Each electromechanical relay will be identified with a device number.  These are actually ANSI device numbers that tell us exactly what function the device serves.  A list of device numbers can be found at . Make a list of each device in your existing panel.  Some relays, such as auxiliary relays may be irrelevant but it is good to capture all the device numbers as they will be used to develop your relay part number(s) later.

Next you need to verify the control power voltage level. This is a very important item prior to ordering the new relays, if you specify the incorrect control voltage, the relay may not power up or worse yet, you could smoke the relay.  Typical power supplies voltage are, 120VAC, 240VAC, 48VDC or 125VDC.  Some of the latest relays can accept all of these voltages however some relays are limited to the specific voltages. Keep in mind that electromechanical relays do not need a power input to function so you may not have a dedicated power source available in your panel.  If this is the case, you will need to pull power into the panel.  If you have a backup battery system available, it is recommended you use this as your source to provide power to your panel in the event of a power outage. Keep in mind when ordering new microprocessor type relays that they can have different ratings with regard to voltage requirements for inputs, output contacts and power supplies so know what’s required for the new system. Output trip contacts should be rated for the devices they are to trip. This could be different than the supplied control voltage especially if a cap trip device is used. Whenever possible a thorough review of all available drawings is required. A lot of the time we do not have that luxury and therefore a review of the nameplates of all device served will be required.

Please follow good safety practices gathering this information as you will likely be exposed to hazardous energy, unless you take power outage to get the data.

One more piece of data you will need is the Current Transformer (CT) secondary output rating.  Again the choices are limited to typically 1 or 5 amperes.  A typical CT ratio is 600:5, in this case the secondary output of the CT is 5 amperes.  This information is critical to assure the relay is configured properly.  Incorrect CT ratio inputs will compromise relay function and accuracy.

Now we have the following information:

A list of ANSI device numbers from our existing protection system

Power supply voltage – 125 VDC

Control Power Voltage 120 VDC

Current Transformer Ratio 600:5

(For the purpose of this discussion we are excluding all communication options)

Now to develop the relay part number.  This is where you could use a secret decoder ring.  While the manufacturers provide good tools for configuring relays, these are complicated devices and putting together a good part number takes some brain power.

To begin developing a part number you will need to determine what you are protecting. This may be obvious but sometimes multiple protection schemes are contained in one panel making it a bit more complicated.  One panel may include a combination of protections such as transformer and feeder protection.  Refer to the information below to determine the relay type.

  • Line Protection – Protects overhead and underground power lines
  • Feeder Protection – Protects cables and bus feeding a load
  • Transformer Protection – Protects Transformers
  • Differential Protection – Protects lines and transformers
  • Motor Protection – Protects motors and the conductors feeding the motor
  • Generator protection – Protects generators

Using your list of device numbers you created earlier will help narrow the choices of relays.  If your list of devices includes 50, 51, 50N, 51N, most likely, you have a basic feeder protection system.  If your list includes all the above and 49, 59 and 87 then you likely have a transformer protection system.  If you are unsure contact any of the major manufacturers, provide them your list of device numbers, and they can point you in the right direction.

In this example we will assume we are working on a transformer protection panel.  Our device numbers are 50, 51, 50N, 51N, 49, 59 and 87, the CT secondary current is 5 amperes, the control voltage and power supplies are 120 VDC. With this information we have everything we need to develop a valid part number for your project. Most manufacturers have an on-line configurator to assist in the process. The configurators provide you all the choices necessary to configure a valid part number.  As mentioned earlier, I have not included any details regarding communication options, we will save that for a later date.  After the relay is configured, you will have part number and a list of features and functions of the relay.

I have created a relay part numbers for a Schweitzer SEL 387 relay.  The data sheet is below.

If you want to give it a try click the links below.  You will see options for all the information we have discussed.

General Electric

Schweitzer Engineering Laboratories

relay upgrades