Power over Ethernet (PoE)

Overview

Power over Ethernet (PoE) is a technology that allows both data and electrical power to be transmitted over a single Ethernet cable.

It is widely used in:

• CCTV systems (IP cameras)
• Access control systems
• Wireless access points
• VoIP phones

For fire and security systems, PoE simplifies installation by:

• reducing the need for local power supplies
• minimizing cabling requirements
• centralising power distribution

How PoE Works

PoE injects DC power onto standard Ethernet cabling (typically Cat5e, Cat6, etc), allowing connected devices to draw power directly from:

• a PoE-enabled network switch (endspan)
• a PoE injector (midspan)

The powered device (PD) negotiates with the power sourcing equipment (PSE) to determine how much power it requires.

PoE Standards

PoE is standardised under the IEEE 802.3 family:

Note: Actual usable power at the device is lower due to cable losses.

Applications in Fire & Security

CCTV (Primary Use Case)

PoE is heavily used for IP CCTV systems:

• Cameras powered directly from switches
• Simplified installation (single cable)
• Centralised UPS-backed power

Higher power cameras (PTZ, IR-heavy units) often require:

• PoE+ or PoE++

Access Control

Used for:

• door controllers
• IP readers
• intercom systems

However, care must be taken where:

• door release mechanisms require higher current
• fail-safe operation is required during power loss

Wireless Infrastructure

Used for:

• access points
• point-to-point links

Power requirements vary significantly depending on:

• transmit power
• antenna configuration

Power Budgeting (Critical)

One of the biggest real-world issues with PoE systems is incorrect power budgeting.

Each PoE switch has a total power budget, which must not be exceeded.

Example

• 24-port PoE switch
• Total power budget: 370 W

This does not mean:

• 24 × 30 W devices can be connected

Instead:

• total load must remain within 370 W

🔧 PoE Calculator

To correctly size PoE systems, use:

Use the Fire Secure UK PoE Budget Calculator to check switch and per-port capacity.

This allows you to:

• calculate total load
• validate switch capacity
• prevent overload conditions
• plan for future expansion

Design Considerations

1. Total Power Budget

Always calculate:

• total device load
• available switch capacity
• spare headroom (recommended)

Failure to do this can result in:

• devices not powering up
• intermittent faults
• system instability

2. Cable Length and Voltage Drop

PoE is affected by cable length:

• standard max = 100 m (Ethernet limit)
• longer runs = increased voltage drop

This can result in:

• devices failing to boot
• reduced performance (especially on high-load devices)

3. Device Power Class

Devices negotiate power, but:

• some devices draw close to maximum
• others spike during startup (e.g. PTZ cameras)

Always design based on worst-case draw, not typical.

4. Centralised Power (UPS Integration)

One of the biggest advantages of PoE:

• entire system can be backed up via a single UPS

However:

• UPS must be sized for full PoE load
• runtime calculations must include all powered devices

5. Redundancy

Critical systems may require:

• dual power supplies on switches
• multiple PoE switches (load distribution)
• network redundancy (ring/topology design)

Common Field Issues

1. Overloading PoE Switches

Very common issue:

• installer counts ports, not power
• system works initially
• fails when all devices draw full load

2. Cheap / Non-Standard PoE Equipment

Not all “PoE” equipment is equal:

• non-compliant injectors
• passive PoE (non-standard voltage)

This can:

• damage devices
• cause inconsistent operation

3. Ignoring Startup Power Draw

Devices such as:

• PTZ cameras
• IR cameras

can draw higher power on startup than during normal operation.

This can:

• trip switch limits
• cause reboot loops

4. Voltage Drop on Long Runs

Long cable runs can cause:

• insufficient voltage at device
• intermittent operation

Especially problematic with:

• PoE+ / PoE++ devices

5. No Headroom in Design

Systems designed at 100% capacity:

• leave no room for expansion
• become unstable under fault conditions

Best practice:

• allow 20–30% spare capacity

Comparison with Traditional Power

Practical Guidance

When designing a PoE system:

1. Calculate total load using the PoE calculator
2. Check switch power budget (not just port count)
3. Allow spare capacity for expansion
4. Consider startup power draw
5. Validate cable lengths
6. Integrate with UPS where required
7. Avoid non-standard PoE equipment

Summary

PoE is a core technology in modern security systems, enabling:

• simplified installation
• centralised power management
• scalable infrastructure

However, its limitations, particularly around power budgeting and cable distance, must be properly understood.

Poor PoE design can lead to:

• unreliable systems
• intermittent faults
• complete device failure under load

Correct design, backed by proper calculation, ensures stable and compliant operation.