Current transformers are often constructed by passing one or more primary turns (either an insulated cable or an uninsulated bus bar) through an insulated toroidal core wrapped with many turns of wire. The CT is typically described by its current ratio from primary to secondary. For example, a 400:5 CT would provide an output current of 5 amperes with a primary current of 400 amperes… in other words: a current of 400 Amps goes in, and 5 Amps comes out.

1. Determine which relay you need your resistor connected to.
2. Determine the current needed for the relay.
3. Determine the initial current rating for the resistor.
4. Select a CT that is rated for the primary and secondary currents.

• CT200
• EFCT
• 5SHT
• W Series
• WAB Series

We can also provide CTs for any custom application, or specs, from any manufacturer. The listed CTs are the most common, and compatible with the ground fault protection relays we commonly use.

In a three phase power system, there are two common configurations. A wye configuration, and a delta configuration. In a delta configuration, there is no common “star point” where each line is connected. In a wye configuration, there is a common star point, and quite often there is also a dedicated neutral line. In order for a Neutral Grounding Resistor to do its job, it must be connected to a Neutral, in order to Ground it through Resistance. A neutral grounding / earthing transformer will either create this neutral connection, or it will change its voltage to make it suitable for usage in the NGR.

There are two main types of earthing / grounding transformers:

• Zig-Zag Transformer:
  • Creates a neutral connection from a delta configuration
  • For more detailed information on HOW Zig-Zag transformers work, visit [our guide].

• Potential Transformer:
  • Changes the supply voltage (line-to-neutral)
  • Also known as a voltage transformer
  • Two sub-types
    • Step-Down Transformer: reduces the supply voltage
    • Step-Up Transformer: increases the supply voltage (exceptionally rare usage)
  • For more detailed information on HOW potential transformers work, visit [our guide].

If you are using a Delta configuration, you MUST use a Zig-Zag transformer to connect your Neutral Grounding Resistor. In addition to the Zig-Zag transformer, you can connect a Potential Transformer to your newly formed neutral connection.

If you already have a Neutral line or connection, why would you want a Potential Transformer?

The simple answer is to enable high resistance grounding systems. For example, if you wanted to take advantage of pulsing systems, but your line-to-neutral voltage is high, it would be exceptionally expensive to create a high resistance system.

You can reduce the line-to-neutral voltage with the use of a potential transformer, specifically a step-down transformer. This voltage will be the new supply voltage of the Neutral Grounding Resistor, and allow you to use high-resistance-only features and be significantly more cost effective.

For more information on high resistance grounding vs. low resistance grounding, read our useful information section below.

We want to bring you the solution that best meets your requirements, and does so most cost-effectively. If you provide us with your specifications, we will deliver this solution. This may mean just the use of a zig-zag transformer, or a potential transformer, or a combination of both with our Neutral Grounding Resistors.

Alternatively, if you already have a visualization of your solution, provide us your specifications and we’ll make sure you have a fully configured system ready to be connected.

Due to regional differences, analogous to Neutral Grounding vs. Neutral Earthing, customers frequently search for earthing transformers, or grounding transformers. These two words refer to the exact same product.

Because we are based in Canada, we frequently refer to these products as grounded, or grounding transformers. Many European and Asian countries will commonly refer to them as an earthing transformer. For short, sometimes these are also referred to as NGR Transformers, or NER transformers.

If you are a customer and are reading this section, you are probably here for one of three reasons.

1. You started out knowing you needed an earthing transformer / grounding transformer for your three phase system, and are looking for transformer grounding. Specifically a transformer resistor.
2. You know you needed a Neutral Grounding Resistor, and need to use a ground transformer in order to use an NGR.
3. You want to understand when and why a Neutral Earthing or Neutral Grounding transformer with an NGR / NER.

In any case, you’ve find the right place.

The above MegaResistors diagram displays how Neutral Grounding Resistors work in a system, and how their functionality can be extended.

1. Neutral Grounding Resistors are used in general to ground Transformers or Generators with Delta or Wye secondaries.
   1. If Delta, a Neutral Grounding Transformer is used to create a Neutral point.
   2. In both cases, a Neutral Grounding Transformer can be used to reduce the voltage applied to the NGR.
2. The NGR is connected to the neutral point and can have additional features such as:
   1. Sensing resistors, allow resistor monitoring.
   2. Bushings, allow for ease of connection with external ground and neutral insulated conductors.
   3. Pulsing resistors, speed up the location of the ground fault.
   4. Other components, such as a disconnect switch.
3. The neutral is passed through a current transformer, which will be connected to a ground fault protection relay.
4. The relay has the primary function of detecting ground faults.
   1. Detected values can be displayed on metering devices (Ammeter, Voltmeters).
   2. If a ground fault is detected…
      1. Low resistance:  The relay will trips the breaker.
      2. High resistance: The relay will activate an alarm, there is no need to trip.
   3. The relay can also send information to the logic controller if using second ground fault protection.
5. Lamps on the control panel provide important information from the relay.
   1. There are numerous functions on the control panel that allow staff to control the system.
   2. One of these functions is the pulsing system, which activates the pulsing resistor.
6. If using second ground fault protection, the logic controller will control and assign priorities to feeders.
   1. It will send commands to the relays to control the system.
   2. It will send information to the control panel to display.

Anything in the low resistance bracket shows what a low resistance system is capable of. Anything in the high resistance bracket shows what a high resistance system is capable of.

These features and components can be enclosed in two separate (resistor and controls) enclosure, or in a single enclosure. Depending on the voltage class, or if using a second ground fault protection system, only separate enclosures are allowed.

Low Resistance Grounding Systems (LRGs) and High Resistance Grounding Systems (HRGs) are both subsets of Neutral Grounding Resistors. The primary difference between them is their Duty Rating, and the functions they can deploy as a result of their duty rating.

LRGs are designed to trip the system within a specific amount of time (duty rating). Most Neutral Grounding Resistors are designed to trip within 10, 30, or 60 seconds. Typically this can allow time to softly shutdown any sensitive equipment to prevent damage, by notifying an immediate area with the sound of a horn.

HRGs are designed to maintain continuous operation of the system without interruption! A ground fault can cause hours of downtime resulting in the loss of hundreds to millions of dollars depeneding on your application, a HRGs prevents this. The duty rating for all HRGs are “Continuous”, and not specified with time.

In addition to the massive advantage of preventing any interruption, HRGs are capable of supporting many useful features. Because the HRGs maintains a relatively low current (5A or 10A), it is capable of using a pulsing system to help you quickly detect the exact location of a ground fault and alleviate the issue. In addition to this, a HRGs is needed for feeder monitoring or second ground fault protection, two very advanced features. For more information on second ground fault protection, click [here].

There is one minor downside to HRGs; supporting very high voltages (above 600V) results in significantly increased costs. If you are interested in maintaining continous operations and using HRGs-only features, you can use a Neutral Grounding Transformer in conjunction.