# Circuit breaker

Circuit breaker is an exchange rule that temporarily pauses trading when a market index or individual stock moves beyond a specified threshold, allowing time for orders and information to be reassessed.

## What it means

A sudden price move can trigger an exchange-mandated pause rather than continuous trading. A circuit breaker may apply to a broad market or to an individual stock. It is a market rule, not an order type or a guarantee that prices will stabilize.

In the US, market-wide circuit breakers use declines in the S&P 500 from its previous closing level. US stocks can also be paused under limit up-limit down (LULD) rules when trading cannot occur within the stock’s applicable price bands. European exchanges have their own volatility-interruption rules. Thresholds, reference prices, pause lengths and reopening procedures depend on the market and rule in force.

A circuit breaker can cause a trading halt, but not every trading halt is a circuit breaker: trading may also stop for news, regulatory reasons or other exchange procedures.

## How it is calculated

For a market-wide decline threshold, the basic measurement is `decline (%) = (reference index level − current index level) / reference index level × 100`.

- **Reference index level** is the index’s specified starting level, measured in index points; for US market-wide circuit breakers, it is the S&P 500’s previous close.
- **Current index level** is the level being assessed, also in index points.
- **Decline** is a percentage, not a number of index points.

The exchange compares that percentage with its applicable thresholds and timing rules. This formula does **not** describe every stock-level circuit breaker: LULD, for example, uses price bands around a reference price rather than a single percentage decline from the previous close.

## Worked example

These numbers are hypothetical and illustrate the calculation, not a current exchange threshold.

1. Suppose the reference index level is **5,000 points** and a rule specifies a **5% decline threshold**.
2. A 5% decline is `5,000 × 0.05 = 250 points`.
3. The corresponding index level is `5,000 − 250 = 4,750 points`.
4. If the current level is 4,750, the measured decline is `(5,000 − 4,750) / 5,000 × 100 = 5%`.

Whether trading pauses at that level depends on the exchange’s actual rule, including its trigger level and time-of-day conditions. An individual stock can also encounter a volatility pause even when the broad index has not reached a market-wide threshold.

## Why it matters before you trade

A pause changes *when* an order can be executed, not necessarily whether an existing order remains in place. During a halt, execution is unavailable in the affected market, and displayed prices or quotes may no longer indicate a price at which an order will fill after reopening.

After trading resumes, the order book, bid-ask spread and available depth may differ substantially from those seen before the pause. A market order can therefore fill at an unexpected price; a limit order sets a price boundary but may remain unfilled or fill only partly. Broker and exchange rules determine how open orders are handled, so order status is worth checking with the broker.

A pre-trade analysis based on earlier market data cannot remove the timing and execution uncertainty created by a sudden halt. Order size, participation in daily volume and liquidity matter especially when trading resumes and many orders compete for available shares.

## How PreTrAIde uses it

PreTrAIde provides pre-trade analysis before the user places an order with their own broker. Its documented quantitative layer assesses estimated spread, participation in daily volume, market impact, volatility and liquidity; its output includes a basket-level and per-stock verdict, risk assessment and suggested execution strategy. A **suspended stock is always NO-GO** under its guardrails.

The product facts do not say that PreTrAIde detects circuit-breaker triggers, monitors live exchange pauses or calculates exchange-specific thresholds. A circuit-breaker check therefore remains a separate, time-sensitive check of exchange and broker information. PreTrAIde does not receive, transmit or execute orders.

## Common mistakes

- **Treating every halt as a circuit breaker.** Other halt reasons exist, and their reopening procedures may differ.
- **Assuming a pause fixes the price.** Trading can resume above or below the last traded price.
- **Using one threshold for every market or stock.** Rules and reference prices differ by venue and instrument.
- **Assuming an unfilled order disappears.** Its status depends on the order terms and broker and exchange rules.
- **Confusing a pause with liquidity.** A reopened market can still have a wide spread or limited depth.

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Source: https://pretraide.com/glossary/circuit-breaker