How to set an SE-330 ground fault relay

The Littelfuse SE-330 is an industry common ground fault protection relay for neutral grounding resistors.

The protection relay provides ANSI protection elements 50G, 50N, 51G, 51N, 59N, and 86. This means instantaneous and time-overcurrent ground and neutral protection, ground fault detection using residual over-voltage measurement (59N), and lock-out relay (86).

The SE-330 can detect the current and resistance of the neutral grounding resistor (NGR).

This article outlines the basics of how to set the SE-330 protection settings.

Step 1: Determine the NGR size

The sizing of the NGR is outside the scope of this article. However, typically the NGR is sized based on the system charging current. System charging current can be thought of as the amount of capacitive current that flows in the event of a ground fault. Typical values for small low-voltage systems are 1A per 1000 kVA. If the system has long cables, it is best to perform a calculation for the system charge current.

To program the SE-330, the NGR ratings are required. Find the nameplate ratings for voltage rating, resistance, and continuous current thermal rating (seconds).

Find the current rating of the NGR using the line-to-neutral voltage and the NGR resistance (using Ohm's law).

$I_{NGR} = \frac{V_{line-to-neutral}}{R_{NGR}}$

Step 2: Calculate ground fault trip setting

A common and generally recommended ground fault setting for an NGR is a definite-time ground fault protection (50G) set at 20% of the NGR current rating. The maximum ground fault current that can be seen on a resistively grounded system is the NGR current rating. 20% of this rating has been found in the industry as a good balance between nuisance tripping and excessive ground fault current.

Example: A system has a 600V, 1000 kVA delta-wye transformer with a 69 Ω neutral grounding resistor. The 600V transformer secondary feeds LC #1. A feeder from LC #1 feeds MCC #1. MCC #1 feeds various 600V motors. What should the SE-330 ground fault (50G) protection setpoint be?

First, find the NGR current rating.

$I_{NGR} = \frac{V_{line-to-neutral}}{R_{NGR}} = \frac{347V}{69Ω} = 5A$

The NGR is rated for 5A on this system.

Using the 20% rule, calculate the pick-up setpoint.

$I_{setpoint} = I_{NGR} * 20\% = 1A$

The SE-330 50G element shall be set to 1A. This aligns with Table 1 from the SE-330 manual.

Step 3: Calculate ground fault time delay

For a 50G protection element (definite time), there is a time-delay setting on the SE-330. It is adjustable from 0.1 to 10 seconds. The time delay should be as short as possible but allow for proper coordination with downstream ground fault protection relays.

To determine the 50G protection element time-delay on the SE-330, we need to start downstream and work the way up to the NGR.

If the motor ground fault is definite time with a delay of 0.3 seconds (determination of this value is outside the scope of this article), the LC #1 feeder ground fault time delay could be 0.5 seconds. This allows for 0.2 seconds for the downstream motor protection to operate. If LC #1 is a 0.5 seconds delay, a typical value for the upstream SE-330 time delay would be 1 second. Note, a protection and coordination study is best advised to determine these values.

Now we have our basic set points for ground fault protection on the NGR: 1A at 1 second.

Step 4: Calculate 59N setpoint (voltage)

There may be a scenario where a ground fault occurs and the NGR fails open. The SE-330 has backup voltage protection for this risk.

In a typical ground fault scenario, the voltage across the NGR is the resistance multiplied by the fault current (Ω's law). See the equation below.

$V_n = I_{fault\_current} * R_{NGR}$

If we use our trip set point from step 3, we can calculate the voltage across the resistor at the pick-up point.

$V_n = I_{trip\_setpoint} * R_{NGR} = 1A * 69Ω = 69V$

In this example, a voltage above 69V indicates we are in the tripping range.

In the case that the resistor fails open, the fault current is zero as there is no ground fault path. We now can only rely on the voltage across the resistor to indicate a ground fault.

The SE-330 advises setting the $V_{n}$ value to the next largest value of calculated. The voltage setpoints are discrete values of 20, 60, 100, 130, 170, 200, 340, 800, 1200, 1700, and 2000.

Example 1

A 480V system has a 55Ω neutral grounding resistor:

$I_{NGR} = \frac{V_{line-to-neutral}}{R_{NGR}} = \frac{\frac{480V}{\sqrt{3}}V}{55Ω} = 5A$

$I_{setpoint} = I_{NGR} * 20\% = 1A$

$V_n = I_{trip\_setpoint} * R_{NGR} = 1A * 55Ω = 55V \:-\!\!> 60V$

Example 2

A 4160V system has a 160Ω resistor:

$I_{NGR} = \frac{V*{line-to-neutral}}{R_{NGR}} = \frac{\frac{4160V}{\sqrt{3}}V}{160Ω} = 15A$

$I_{setpoint} = I_{NGR} * 20\% = 3A$

$V_n = I_{trip\_setpoint} * R_{NGR} = 3A * 160Ω = 480V \:-\!\!> 800V$

Example 3

A 13,800V system has a 319Ω resistor:

$I_{NGR} = \frac{V*{line-to-neutral}}{R_{NGR}} = \frac{\frac{13800V}{\sqrt{3}}V}{319Ω} = 25A$

$I_{setpoint} = I_{NGR} * 20\% = 5A$

$V_n = I_{trip\_setpoint} * R_{NGR} = 5A * 319Ω = 1595V \:-\!\!> 2000V$

Note, when the S5 switch on the SE-330 is 100-kΩ, multiply the $V_{n}$ dial by 5. e.g. 2000V is 10,000V.

Disclaimer: This article is for informational purposes only and is not engineering advice. Consult a professional engineer for your specific application.

This article was written using the Littelfuse SE-330 manual revision 11-A-063018.