Hot ground is the local circuit reference on the non-isolated, mains-derived side of a power supply; cold ground is usually the reference on its isolated secondary side. They are not two kinds of protective earth, and neither label alone proves a point is safe to touch. The distinction matters most when identifying circuit sections or connecting test equipment.
What “ground” means here
In a circuit, ground often means a chosen voltage reference and return path. It does not necessarily mean the node is connected to the earth beneath your feet. The word is used for several different connections:
- Circuit ground: a reference or current-return node within a circuit.
- Signal ground: the reference used by analog or digital signals.
- Chassis ground: a connection to an equipment enclosure or frame, if the design provides one.
- Protective earth (PE): the safety conductor intended to carry fault current and help keep accessible metal near earth potential.
- Mains neutral: a current-carrying supply conductor; it is not interchangeable with protective earth.
- Hot ground and cold ground: informal service terms for local references on opposite sides of an isolation barrier.
A ground symbol on a schematic identifies a circuit reference, not a promise of earth potential or safety. Tektronix explains how an oscilloscope probe’s ground connection relates to the instrument’s grounding system in its probe primer.
Where hot and cold ground are in a typical switching supply
In an isolated offline switching power supply, the input side is the primary and the output side is the secondary. Energy crosses the transformer magnetically; the circuit references on the two sides are not ordinarily joined by a direct conductive path. Service documentation illustrates the hot-primary/cold-secondary distinction, including the Philips DVD recorder service manual and a Philips television service manual.
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A simplified layout looks like this:
AC input → bridge rectifier → primary bulk capacitor and switching circuit
║ isolation barrier ║
transformer → secondary rectifier → output rails
Primary return: hot ground Secondary return: cold ground
The transformer and any optocoupler or other isolation-rated feedback component can help you locate the barrier on a schematic. The actual design may differ, so use the device’s schematic and board markings rather than this simplified layout alone.
Hot ground: the primary-side reference
Hot ground usually refers to the return reference around the AC rectifier, primary bulk capacitor, switching transistor, primary controller, startup and snubber circuitry, and current-sense network. In many supplies it is the negative terminal of the primary bulk capacitor, but that is not a universal rule. “Negative” or “GND” on the primary does not mean safe, and other primary nodes can also be hazardous.
The primary is mains-derived and may be at a dangerous voltage relative to earth. For a rough illustration, the peak of an ideal 120 V RMS sine wave is about 170 V, using Vpeak ≈ Vrms × 1.414; a rectifier charges a bulk capacitor toward the AC peak, subject to circuit details and line variation. The capacitor can retain hazardous charge after the unit is unplugged.
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Cold ground: the secondary-side reference
Cold ground usually means the return reference for the isolated output: secondary rectifier and filter returns, low-voltage rails, regulators, and often audio, video, logic, USB, or control circuitry. Depending on the design, it may also connect to signal ground or chassis. Those connections must be checked in the particular product, not assumed.
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“Cold” describes the side’s intended isolation relationship to the primary. It does not mean zero volts to earth, low voltage in every design, or safe to touch. An isolated output can itself be high voltage; capacitors can store energy; leakage through safety components can produce a meter reading; and a fault or incorrect repair can compromise isolation. CRT equipment has additional high-voltage hazards.
How to identify the two references
- Find the isolation boundary. On the schematic, locate the switching transformer and any optocoupler or other isolation-rated feedback path. Look for separate primary and secondary ground symbols or labels.
- Identify the primary-side parts. The AC input, bridge rectifier, primary bulk capacitor, and switching transistor are usually on the hot side. The bulk-capacitor negative is often the local primary return, not protective earth.
- Identify the secondary-side parts. Follow the transformer secondary winding to its rectifier, filter capacitors, and output rails to find the local secondary return.
- Check the service manual and board markings. Some products explicitly mark hot and cold ground. Do not rely only on wire color, a “GND” label, or a negative sign.
The negative terminal of the primary capacitor and the negative terminal of a secondary output can both appear to be “ground,” yet remain separate references. Joining them casually can defeat the intended isolation or cause damage.
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Why the isolation barrier matters—and where it may not be enough
Isolation transfers energy without a direct conductive connection between the primary and secondary. It helps keep accessible low-voltage circuitry from being directly tied to mains and lets the secondary use its own reference. Safety depends on the complete design and its condition, however. EMI-suppression capacitors, shields, chassis connections, faults, and incorrect repairs can create paths that affect measurements or compromise protection.
Some converters are non-isolated. In a buck or boost supply without galvanic isolation, the output may remain mains-referenced, so there may be no genuinely cold ground. Autotransformers also do not provide galvanic isolation. Double-insulated equipment may lack a protective-earth conductor while still containing hazardous primary circuitry; battery-powered equipment can float relative to earth without being harmless. USB or HDMI connections to other grounded equipment can change the reference relationships in a setup.
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Using an oscilloscope without creating a short
A conventional bench oscilloscope’s probe ground lead is normally connected to the scope’s grounding system and protective earth. Clip it to a mains-derived primary node and the probe lead can provide a low-impedance path to earth. That may short the circuit, damage the probe, scope, components, or traces, and expose someone to a shock or arc. The waveform may also be invalid because the measurement has changed the circuit.
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For a secondary-side measurement
A standard passive probe may be appropriate for a low-voltage secondary measurement when the secondary is genuinely isolated, the reference point is suitable, and the circuit voltage and measurement category are within the ratings of the probe and scope. For ripple, the probe ground is commonly connected to secondary return. Keep the ground connection short where possible: a long lead can add inductance and create ringing or false spikes. Check bandwidth, input limits, and the manufacturer’s instructions before measuring.
For a primary-side or floating measurement
Do not attach an ordinary grounded probe’s ground clip to hot ground, a switching node, or another point that is not meant to be earth-referenced. Use a correctly rated differential or isolated measurement system, such as a high-voltage differential probe, optically isolated differential probe, manufacturer-specified isolated scope input, or battery-powered isolated instrument within its stated limits. Tektronix discusses differential techniques for power-converter measurements in its application note; Keysight describes optically isolated differential probes in its probe datasheet.
Choose equipment by its maximum differential voltage, common-mode voltage, CAT rating, bandwidth, input impedance, and clearances—not bandwidth alone. A differential probe measures the voltage between its input points, but both points may still sit at a dangerous voltage relative to earth. Check both the differential and common-mode limits. Common-mode rejection also varies with frequency and setup.
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Do not lift the scope’s protective earth
Never remove the oscilloscope’s earth pin, use a cheater plug, or disconnect its protective conductor to make the instrument “float.” Tektronix warns that defeating the scope’s protective grounding can put hazardous voltage on its chassis and accessible connectors in its operator-protection brief.
Two ordinary probes with their channels subtracted are not a casual substitute for a properly rated differential probe: both ground clips remain earth-connected, and the arrangement can exceed ratings or provide inadequate common-mode rejection. Tektronix lists MOSFET drain-source voltage, transformer voltage, diode voltage, and voltage across ungrounded resistors among typical differential-measurement applications.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What an isolation transformer does—and does not do
An appropriately rated isolation transformer can isolate the device under test from the utility supply as part of a controlled service setup. It does not discharge capacitors, remove high voltage, eliminate arc hazards, or make a live primary safe to touch. It is also not a safe way to float a grounded oscilloscope: keep the scope’s protective earth intact and use suitable differential or isolated measurement equipment instead.
Legacy televisions and “hot chassis”
Older television and radio service literature may describe a hot chassis connected directly or indirectly to a mains-derived circuit, or a cold chassis separated from the line by a transformer. Some designs had distinct hot and cold grounds in different sections. Historical service material documents these arrangements, including this 1979 Electronic Servicing article.
Those labels are not a safety certification. A cold chassis does not remove CRT anode, flyback, or stored-capacitor hazards, and a hot chassis must be treated as potentially live. Do not assume the chassis or a point marked ground is safe without checking the specific service documentation and circuit.
Common mistakes and safer decisions
- Assuming every ground symbol means earth: identify whether it is a local circuit reference, chassis connection, or protective earth.
- Assuming every negative terminal is the same: primary and secondary returns may be isolated from one another.
- Connecting a scope ground clip before checking the circuit: that connection can short a primary-side node to earth.
- Trusting an isolation transformer as complete protection: it changes an electrical relationship; it does not remove stored energy or make exposed live work safe.
- Ignoring common-mode voltage: a small voltage between probe inputs does not mean either input is safe relative to earth.
- Relying on a meter reading alone: high-impedance meters can show AC voltage coupled through EMI capacitors, while unsuitable references or transient conditions can mislead. A reading is not permission to touch.
For trained technicians, the safe sequence is to identify the barrier and relevant reference from the schematic, de-energize and discharge capacitors using the manufacturer’s procedure, then verify absence of voltage with a suitably rated meter. Use appropriate insulated tools and PPE, and energize only when a live measurement is necessary and the setup is controlled. Select probes within all stated electrical and physical limits. If you are not trained to work on energized mains-powered equipment, do not open or probe it; unplugging the unit is not enough to ensure capacitors are discharged.
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