PCB for Relay: Isolation and Clearance Requirements

Relay PCBs interface low-voltage control circuits with high-voltage mains power (230V AC in India). The critical design challenge is maintaining safe electrical isolation between the control side and the load side to prevent electric shock, arcing, and fire. This guide covers creepage and clearance distances, relay selection, PCB layout rules, and safety certification requirements for Indian relay-based products like home automation switches, industrial controllers, and IoT smart plugs.

Table of Contents

• Safety Isolation Basics
• Clearance and Creepage
• Relay Selection
• PCB Layout Rules
• Isolation Slots and Cutouts
• Hi-Pot Testing
• Indian Safety Certification
• Frequently Asked Questions

Safety Isolation Basics

When a PCB carries both mains voltage (230V AC) and low-voltage control (3.3V/5V), the isolation between them protects users from electric shock. The isolation must withstand:

• Working voltage: 230V AC RMS (325V peak)
• Transient overvoltage: Up to 2.5kV for Overvoltage Category II (typical for plug-in equipment)
• Dielectric withstand: 1.5kV RMS for 1 minute (routine production test)

Safety standards define minimum distances based on the voltage and pollution degree of the environment.

Clearance and Creepage

For Indian residential products (230V AC, Pollution Degree 2), maintain at least 5mm creepage and 3mm clearance between mains and low-voltage conductors on the PCB. For reinforced insulation (required when users can touch the low-voltage side), double these values.

Relay Selection

For IoT/home automation products in India, use relays from reputable manufacturers (Omron, Hongfa, Songle) with UL/TUV listings. Using uncertified relays makes safety certification of your product impossible.

PCB Layout Rules

• Physical separation: Divide the PCB into a mains zone and a low-voltage zone. All mains traces, relay contacts, and terminal blocks are in the mains zone. All MCU, WiFi module, and sensor circuits are in the low-voltage zone
• No copper pour crossing the boundary: The ground plane must not extend from the low-voltage side to the mains side. Each side has its own isolated ground
• Wide mains traces: For 10A at 230V AC, use minimum 2.5mm trace width on 2oz copper. Wider is better for thermal performance
• No traces between relay pins: The relay’s coil and contact pins are on opposite sides — do not route any trace between the coil pins and the contact pins on the PCB
• Keep mains traces away from edges: Minimum 3mm from board edge for mains voltage traces

Isolation Slots and Cutouts

To achieve adequate creepage distances on compact PCBs, cut isolation slots in the FR-4:

• A slot (routed cut-through) in the PCB forces the creepage path to go around the slot, dramatically increasing the effective creepage distance
• Typical slot width: 1-2mm (matches the router bit diameter)
• Slot length: extend across the full boundary between mains and low-voltage zones
• Leave bridge tabs at the slot ends for mechanical integrity, maintaining the required creepage at the tab
• Specify slots in your board outline Gerber layer and in the fabrication drawing

Hi-Pot Testing

Every mains-connected product must pass a dielectric withstand (Hi-Pot) test during production:

• Test voltage: 1.5kV AC RMS for 1 minute (basic insulation, 230V working voltage)
• Pass criteria: Leakage current below 1mA during the test
• Equipment: Hi-Pot testers from Chroma, Associated Research, or budget models from ₹15,000
• Test points: Apply voltage between all mains-connected conductors (shorted together) and all low-voltage conductors (shorted together)

Design your PCB with test points on both sides of the isolation barrier for easy Hi-Pot testing during production.

Indian Safety Certification

• BIS (Bureau of Indian Standards): Mandatory for products connected to mains power in India. Covers safety (IS 302-2-series for appliances, IS 616 for household switches)
• BIS testing: Conducted at NABL-accredited labs (ERTL, TUV India, UL India). Costs ₹50,000-3,00,000 depending on product category
• CE marking: Required for EU export. LVD (Low Voltage Directive) and EMC Directive compliance
• IEC 60335-1: Base safety standard for household appliances. Your product’s safety design must meet this standard’s isolation requirements

Frequently Asked Questions

Can I use a solid-state relay (SSR) instead of a mechanical relay?

Yes, SSRs have no moving parts (longer life), no contact bounce, and are silent. However, SSRs have higher on-state voltage drop (1-2V vs millivolts for mechanical relay), generate more heat at high currents, and can leak current in the off state. For resistive loads (heaters, lights), SSRs work well. For motor and inductive loads, mechanical relays are more robust.

What is the minimum PCB thickness for mains voltage?

Standard 1.6mm FR-4 is adequate for 230V AC applications when clearance and creepage rules are followed. The through-thickness dielectric strength of FR-4 is approximately 20kV/mm, providing a large safety margin. However, isolation slots in the PCB improve safety by preventing surface tracking.

How do I drive a 5V relay from a 3.3V MCU?

Use an NPN transistor (BC547) or N-channel MOSFET (2N7002) to switch the relay coil. Connect a flyback diode (1N4148) across the relay coil to protect the transistor from the inductive voltage spike when the coil turns off. The MCU GPIO drives the transistor base/gate through a 1-10kΩ resistor.

Browse PCB prototyping boards, soldering tools, and electronics supplies at Zbotic PCB & Prototyping — fast shipping across India.