UPS earthing

Why You Should Consider Earthing Your Uninterruptible Power Supply

The reliability of your uninterruptible power supply (UPS) is determined by your earthing (AKA grounding) system. Without a properly installed earthing system, you risk your UPS malfunctioning. 

What is an uninterruptible power supply (UPS)?

An uninterruptible power supply provides a continuous flow of electricity to a device, even during a power failure or dips in connectivity. An uninterruptible power supply will also improve the quality of power offered to the critical load, ensuring all your data is kept secure. 

What is an earthing system?

In an electricity supply system – such as a UPS – an earthing system connects specific parts of that installation with the Earth’s conductive surface for safety.

Why is UPS earthing so important?

For uninterruptible power supply to function correctly, it’s vital that a suitable earthing system is installed. The UPS earthing system will allow the circuit protection to clear a ground fault, thereby creating a path to divert the surge current away from a UPS. This will also remove the ‘undesirable current’ from the critical load. The earthing techniques used will depend on the configuration of the UPS and the type of electrical supply installed.

What is the electrical supply for your UPS? 

As mentioned above, the earthing of your uninterruptible power supply depends on the type of electrical supply. There are four common types:

  • TN-C-S: This is the most common type. It uses Protective Multiple Earthing (PME) whereby the neutral and earthed protective conductors are part of the system.
  • TN-C: The neutral and earthed conductors are combined throughout the system. This type also uses PME.
  • TN-S: The neutral and earthed conductors remain separate throughout the system.
  • TT: Neutral is earthed at the power source but the electricity supplier does not provide an earthed protective conductor.

Earthing for different UPS types

The configuration of the uninterruptible power supply is also important when it comes to determining the earthing system. Here are the configurations:

1. Transformer-based UPS or transformer-less UPS with bypass

The UPS neutral should not be bonded to the grounding conductor. This earthing configuration:

  • Doesn’t change the upstream and downstream earthing system;
  • Doesn’t provide any galvanic isolation; and
  • Doesn’t reduce common-mode noise.

2. Transformer-based UPS without bypass

The UPS neutral should be bonded to the grounding conductor. This earthing configuration will:

  • Change the upstream and downstream earthing system;
  • Provide galvanic isolation; and
  • Reduce common-mode noise.

3. Transformer-less UPS without bypass

The UPS neutral should not be bonded to the earthing conductor. This configuration:

  • Doesn’t change the upstream and downstream earthing system;
  • Doesn’t provide any galvanic isolation; and
  • Doesn’t reduce common-mode noise.

4. UPS with isolated bypass

A transformer is introduced in the UPS bypass path. The output neutral of the UPS should be bonded to the earthing conductor and the UPS acts as a separately-derived source. This configuration:

  • Change’s the upstream and downstream earthing system;
  • Provides galvanic isolation; and
  • Reduces common-mode noise.

Earthing for generators within a UPS

It is also important to note the neutral connections around generators within an uninterruptible power supply system. UPS systems and generators rely on breaking the neutral when switching to and from the generator supply, but this relies on the UPS being able to withstand a break in supply neutral during the changeover. There are two main switching mechanisms for UPS generators:

  • One which uses two electrically interlocked contractors controlled by an Automatic Mains Failure; 
  • The other is a make-before-break contact method that ensures the neutral stays fixed during the changeover. 

Things to note when earthing a UPS

  • Do not use 4 pole breakers at the input because this breaks the input neutral, resulting in the floating of output neutral.
  • Do not earth the output neutral of the UPS unless there is a transformer in the bypass or global input.
  • You must supply both the bypass mains and the rectifier mains with power from the same earthing system. If the earthing systems are different, a transformer is needed. 
  • Do not install multiple earthing in a TNS system, the neutral earthing can only be done at one source.

If you have invested in uninterruptible power supply and are interested in installing the necessary earthing system, then it is beneficial to consult with industry experts. They will be able to assess the configuration of the UPS and the best system for earthing installation. There are many different types of UPS devices so it’s important that you know exactly which earthing system is most effective.

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