What Is a Circuit Breaker?
Circuit breaker is a rule that pauses trading across a market, or in a single security, once price falls by a set percentage within a defined period. The thresholds are published in advance, and while a halt is in force no orders execute, including stop orders already resting.
A circuit breaker stops trading when price falls far enough, fast enough. The thresholds are public and mechanical, and the interesting part is not the pause itself but what it does to the orders already resting when it starts.
How it works
A threshold is defined in percentage terms. Market-wide breakers reference a broad index; single-stock versions reference the instrument’s own recent price.
Crossing the threshold triggers a pause. Trading in the affected market or security stops for a defined period, and no executions take place during it.
Then trading resumes, usually via an auction. Orders are collected and a reopening price is established, which may be some distance from the last traded price.
What a pause is meant to achieve
The stated purpose is information. A fast decline can outrun everybody’s ability to work out what is happening, and a pause lets participants read the news rather than react to the tape.
It also restores two-sided quoting. Liquidity providers withdraw in a disorderly market, and an auction brings resting interest back together in one place at one time.
What it cannot do is change the news. If the decline reflects something real, the pause delays the adjustment rather than preventing it, and the reopening reflects that.
A worked example
Imagine a position with a stop resting below the market. Price falls quickly, the threshold is crossed, and trading halts before the stop is reached.
During the halt the stop does nothing. It remains an instruction with no venue to execute against, and the position is held for the duration whether you want it or not.
At the reopening the stop becomes live again. It converts to a market order and fills at whatever the auction produced, which may be well below where it was placed.
The stop worked exactly as designed. It promised an order, not a price, and the halt is the clearest demonstration of the difference between those two things.
Market-wide versus single-stock
Market-wide breakers reference a broad index. They are tiered, with successive thresholds producing longer pauses and, at the deepest level, a close for the remainder of the session.
Single-stock versions are narrower. They pause one security when its price moves outside a band around its recent trading, and they trigger far more often.
Both are published rules, not discretionary calls. The percentages, the reference prices and the durations are set out in advance and applied mechanically.
A third category is the regulatory halt. An exchange can stop trading in one security pending news - a merger announcement, a restatement, an investigation - and that halt is about information rather than about speed, so it can last far longer than a volatility pause.
Which makes them one of the few genuinely knowable things in a fast market. You can read the thresholds before you need them, and the time to do that is not while a position is halted.
The original data
On this site’s shared series: median bar range 0.493, ninetieth percentile 1.101, largest bar 2.338. Direction runs average 2.01 bars with a longest of 11. A round trip costs 0.0098, about 2% of the median bar range.
Compare the largest bar to the median. A 2.338 bar against a 0.493 median is a move nearly five times the typical one, and a gap of that proportion is the environment in which halts and bad stop fills belong to the same event.
And the drawdown measurement is the relevant scale: on this site’s series 95% of bars sit below a prior peak, the deepest drawdown is 3.76%, and the longest stretch below a peak ran 73 bars before finishing 3.61% higher. Most declines are survived by waiting rather than by acting during them.
What to do about it before it happens
Know the thresholds for what you trade. They are published by the exchange, and reading them takes minutes on a quiet day.
Size positions for a gap, not for a stop. If the worst case is a reopening well past your level, the position size that survives it is smaller than the one your stop distance implies.
Do not treat a halt as information about direction. It says a threshold was crossed, which is a statement about speed, and it is triggered by rises as well as falls in the single-stock version.
And decide in advance what you will do at the reopening. The auction is a moment of thin, uncertain pricing, and a decision made inside it is a decision made under the worst conditions the session offers.
When it fails
The characteristic failure is discovering the mechanism while inside it. A position is held, trading stops, and there is nothing to do but wait - which is the first time many people learn that a resting stop is not protection during a halt.
The reopening then delivers the second surprise. The auction price can sit far below the last trade, so the stop fills at a level nobody chose, and the loss is larger than the stop distance suggested it could be.
A second failure is assuming a halt means bad news. Single-stock breakers fire on rapid rises too.
A third is placing new orders during the pause in the belief they will execute at the pre-halt price.
A fourth is trading the reopening auction, which is the thinnest and least reliable pricing of the session.
And a fifth is expecting the pause to have fixed something. It bought time to read; whatever caused the move is unchanged when trading restarts.
Related
Volatility covers the speed that triggers a breaker. Stop-loss covers the order a halt suspends. And liquidity covers what withdraws during the move and returns at the auction.
Most people learn this mechanism exists at the worst possible moment - during a halt, while holding something. The part that catches people out is not the pause, it is what happens either side of it: you cannot act during the halt, and the reopening price bears no obligation to be near the price at which trading stopped.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.