What is a power surge, and where does it come from?

A power surge, in plain terms

In a home, voltage has a normal range: for a 120 V supply, Hydro-Québec puts it between 110 and 125 V at the connection point. Hydro-Québec, for its part, uses the term “transient surges”. In the worst case, measured in an open circuit, their peak is generally limited to 6 kV, or 6,000 V, inside a building, and to 10 kV or 20 kV outdoors. In practice, Hydro-Québec adds, these values are considerably reduced by the connected appliances and their protective devices.

A surge can start far from your home, on the grid, or inside the building itself. It loses strength along the way: transient surges drop off quickly as the distance between their source and the appliance increases.

Where power surges come from

The sources fall into two groups: external ones, such as utility switching and lightning, and internal ones, such as large loads switching on or off. This breakdown comes from a technical sheet by the Government of Alberta, written for its public buildings.

Lightning

Hydro-Québec gives lightning as an example of a cause of power surges. In its document on voltage characteristics, it lists lightning among the sources of transient surges. Lightning doesn’t have to hit your house: a direct strike on the power grid can produce large, high-energy surges, according to the Alberta sheet. The farther away the strike, the more the surge weakens before it reaches your appliances.

Switching on the grid

According to Hydro-Québec, switching operations on grid lines and equipment can produce transient surges, including the switching of capacitor banks. The Alberta sheet also lists utility switching among the external causes.

Power coming back after an outage

The moment the power comes back is a risky one: according to Hydro-Québec, the surges that come with it can damage your appliances and can even cause a fire. The Government of Québec puts it another way: unplugging electronics during the outage prevents a surge to your devices when the power returns. The Government of Canada also advises turning down the thermostat during the outage, to avoid surge damage when power is restored.

Large appliances in the house

Not all surges come from the grid: large loads such as motors switching on or off also create them inside the building. Hydro-Québec lists, among the causes of voltage variations, a customer using an appliance that can disturb the electrical wave or cause a sudden inrush of current. It also lists motor starting and sudden load changes among the causes of voltage dips that originate in the customer’s installation. Hence its advice: don’t plug electronics into the same circuit as an appliance with a motor, such as a refrigerator.

Surge, overload, short circuit: three different problems

These three words don’t describe the same thing.

• An overload: the circuit is drawing more than it is rated for, for example when a space heater is added to what is already running on the same circuit.
• A short circuit: a fault in the wiring or in an appliance.
• A surge: the incoming voltage spikes above its normal range.

A breaker, as Hydro-Québec sums it up, automatically cuts the power when there is an overload or another fault. Against surges, Hydro-Québec points to appropriate devices, such as surge protectors.

A breaker that keeps tripping has causes of its own, explained in our guide Breaker keeps tripping: the causes and what to do.

What a surge damages in a house

According to Hydro-Québec, a transient surge can cause an electrical discharge through the insulation of equipment or through electronic components. The Electrical Safety Authority, Ontario’s electrical safety body, adds that a surge can damage a home’s electrical equipment and create a hazard.

Appliances are not the only things at risk: outlets, breakers and connections in the electrical panel can be affected too.

An appliance won’t restart after an outage or a storm? What comes next (safety, proof, claims, insurance) is in our guide Appliances damaged by a power surge.

Storm or outage: what lowers the risk

During a thunderstorm, the Government of Canada advises unplugging appliances to protect them from power surges. Hydro-Québec asks customers to unplug electronics during weather events, outages and planned interruptions for grid maintenance and repairs.

During an outage:

1. Unplug electronic devices, such as computers, televisions and chargers.
2. Leave the refrigerator and the freezer plugged in: Hydro-Québec asks you not to unplug them.
3. Turn down the thermostat, to avoid surge damage when the power comes back.
4. Keep your devices on power bars equipped with surge protectors, which Hydro-Québec recommends against surges when power is restored.

For thunderstorms, the Government of Canada also advises using surge protectors.

Protection at the panel and at the outlet

Hydro-Québec does not guarantee that voltage will stay stable. Its Conditions of Service state that it is up to the customer to guard against voltage variations and to protect their installation and appliances. Hydro-Québec repeats it to its customers: protecting appliances is their responsibility, notably by using appropriate devices.

At the panel. The Government of Canada advises considering a whole-house surge protector. In its words, this helps prevent damage to appliances such as the refrigerator, the stove or the microwave, and it offers much more protection than a simple power bar with surge protection. This kind of surge protector is permanently connected: the CSA standard for Type 2 devices, for example, covers installation on the load side of the main overcurrent protective device. In Québec, this is an electrician’s job: a licence is always required for electrical installation work, even in your own home. Installing unapproved electrical equipment is also prohibited. The surge protector diverts the surge current away from your appliances and into the ground: hence the importance of good grounding (in French), which Hydro-Québec advises having inspected by a master electrician. Where it goes and how the installation works: see our guide Whole-house surge protector, installed at the panel.

At the outlet. Hydro-Québec advises plugging electronics into a power bar with a surge protector that meets CSA or ULC standards. Many power bars with a built-in breaker also include surge protection. How to tell a power strip from a surge protector, the limits of protection at the outlet and why grounding matters: see our guide Power strip, surge protector or panel protection?

When to call an electrician

After a storm or an outage, call right away if you notice:

• a burning smell or unusual heat;
• a hot, blackened or crackling outlet or switch;
• a breaker that keeps tripping, or an outlet that heats up;
• lights dimming or flickering throughout the house at the same time, not just for a second when an appliance starts;
• an electrical panel giving off unusual heat.

Smell, heat or crackling: switch off the main breaker if you can do so safely, move away and call us. If there is smoke or fire, get out and call 911.

Call us at 438-542-7304; we’ll tell you if Hydro-Québec needs to step in. Emergencies are handled by phone, 24 hours a day, 7 days a week. We get to you in 90 minutes or less, anywhere in our service area.

The Government of Québec says it too: for any work involving electricity, including power drops and an unusual smell, call a master electrician. All emergency situations are on our page 24/7 emergency electrician.

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