Power isolation is a critical requirement when choosing a PoE Ethernet Switch. There are many different types of Industrial Power over Ethernet (PoE) Switches on the market. PoE Switches while all having the same name can have vastly different performance, functionality and critically safety.

Over the years Brainboxes has looked at many PoE Switches and found dubious compromises made by a small number of other manufacturers, often for cost reasons. One such compromise being some brands DO NOT isolate the power between the PoE lines and the rest of the device.

What it means

Without power isolation a Power over Ethernet device:

• doesn’t follow the PoE specification
  • IEEE 802.3af/at
  • the IEEE spec mandates that the PoE power should be isolated for the other power domains
    so technically it’s not a PoE device
• doesn’t meet UL standards
  • specifically IEC 62368-1 IEEE 802.3 Clause 33.4.1
  • so shouldn’t have UL certification, however we do see some devices with UL accreditation which don’t really meet the spec
  • by UL we are using a short-hand for Nationally Recognised Test Lab (NRTL) as recognised in America or IEC – the International Standards Body – as recognised world-wide

Why isolation matters

Power over Ethernet Switches without Power Isolation are not safe

• they are susceptible to electrically interference through both the ethernet cables (which could travel up to 100m outdoors) and the power supply
• and susceptible to ground loops (see below)
• therefore there’s a higher likelihood, particularly in industrial environments, of equipment damage; to the switch itself and potentially to other powered equipment

What are Ground loops?

A ground loop is return current which is following a unintended path, (often a long path) which is external to the electrical system. This often results in increased noise emissions, which can be very difficult to track down. In the world of Audio Visual Systems this might manifest as a background ‘mains hum’ which can be heard on the speaker system.

The following diagram shows a multi-port PSE (Power Sourcing Equipment) in this case a PoE Ethernet Switches connected to multiple ethernet devices around a building. Critically the PSE in the diagram below does not have Isolation. When all the ethernet devices in the system are connected to ‘earth’ of the building – in this case the steel structure – then an unintended ground loop is formed and the return current runs through the buildings steel! This is not a good result, ground currents are a major source of noise, hum and interference.

This diagram is taken from slide 6 of this IEEE presentation “802.3da Consequences of Being Grounded”.

What is EMI?

Electro Magnetic Interference EMI, are all sources of unwanted electromagnetic interference including radiated and conducted emissions.

Devices which are accredited to standards such as CE are required to not generate too much EMI and also reliably cope with external sources of EMI. In industrial environments where brainboxes equipment is often deployed being able to reliable cope with EMI is critical.

Brainboxes devices have power isolation

All brainboxes PoE ethernet switches have galvanic isolation between the PoE and the other power domains of 2kV

• we follow the PoE standards IEEE 802.3af/at
• we have UL accreditation (through MetLabs an NRTL) including conforming to IEC 62368-1 IEEE 802.3 Clause 33.4.1
• to do this takes more components, and costs more money, but it makes our customers’ investment safe

What does the circuitry look like

Looking at brainboxes SW-135

We can split the image up into different power domains:

Its the job of the PoE Isolation Transformer to ensure the power available to the PoE lines completely isolated from the power in the rest of the device. The isolation barrier of the transformer keeps these 2 power domains separate; red (PoE power domain) and blue (protected digital 3v3 power domain). This barrier is the same strength as the isolation barrier within the ethernet magnetics 2000 Volts.

Look for a PoE Isolation Transformer on your PoE Ethernet Switch.

In addition any equipment also connected to the same power supply as the device is protected from electromagnetic interference (EMI) and ground loops thanks to the isolation.

See it in action

You can see this in action in 2025 as we are exhibiting around the world with our AI powered PoE vision system demo.

References

IEEE presentation – Consequences of being grounded

https://electronics.stackexchange.com/questions/496031/when-is-it-allowed-to-use-non-isolated-poe

https://electronics.stackexchange.com/questions/698628/poe-pse-isolation-requirements

https://en.wikipedia.org/wiki/IEEE_802.3