Understand circuit filters and ban periods in F&O

Overview

Circuit filters and F&O ban periods are regulatory safeguards implemented by SEBI and exchanges (NSE/BSE) to prevent extreme price volatility and excessive speculation. They act as speed bumps that restrict trading when prices move too rapidly or when derivative positions become dangerously large.

Circuit Filters (Price Bands)

Circuit Filter Categories

Indian exchanges use a multi-tier system:

Lower Circuit Limit: $P_{\text{lower}} = P_{\text{prev}} \times \left(1 - \frac{CF}{100}\right)$

Where:

• $P_{\text{prev}}$ = Previous day's closing price
• $CF$ = Circuit filter percentage (5, 10, or 20)

Derivation from first principles:

The circuit filter concept is based on percentage change limits. Start with the definition of percentage change:

$\text{Percentage Change} = \frac{P_{\text{current}} - P_{\text{prev}}}{P_{\text{prev}}} \times 100$

For a circuit limit of $CF\%$, we want to find the maximum allowable price:

$CF = \frac{P_{\text{upper}} - P_{\text{prev}}}{P_{\text{prev}}} \times 100$

Solving for $P_{\text{upper}}$: $\frac{CF}{100} = \frac{P_{\text{upper}} - P_{\text{prev}}}{P_{\text{prev}}}$

$\frac{CF}{100} = \frac{P_{\text{upper}}}{P_{\text{prev}}} - 1$

$P_{\text{upper}} = P_{\text{prev}} \times \left(1 + \frac{CF}{100}\right)$

Similarly for lower circuit: $P_{\text{lower}} = P_{\text{prev}} \times \left(1 - \frac{CF}{100}\right)$

Step 1: Calculate upper circuit $P_{\text{upper}} = 2500 \times (1 + 0.20) = 2500 \times 1.20 = ₹3,000$

Why this step? We're applying the 20% increase to find the maximum allowed price.

Step 2: Calculate lower circuit $P_{\text{lower}} = 2500 \times (1 - 0.20) = 2500 \times 0.80 = ₹2,000$

Why this step? We're applying the 20% decrease to find the minimum allowed price.

Result: Reliance can trade between ₹2,000 and ₹3,000 today. Orders outside this band are rejected.

Step 1: Check if opening is within circuit

• Upper limit: $950 \times 1.10 = ₹1,045$
• Lower limit: $950 \times 0.90 = ₹855$
• Opening at ₹1,000 is within limits ✓

Step 2: Stock rises rapidly to ₹1,045 at 10:30 AM Why this matters? It reached the upper circuit — the price cannot go higher today.

Step 3: Trading continues, but only at or below ₹1,045 Why this step? For individual stocks, no cooling-off halt occurs — buy orders above ₹1,045 are simply rejected. Sellers willing to sell at/below ₹1,045 can still trade.

Step 4: Contrast with index circuits Why? Only a market-wide index circuit (e.g. Nifty moving ±10%) triggers an actual exchange-wide trading halt for a cooling-off period.

Key Exceptions

Exception list:

1. Index derivatives (Nifty 50, Bank Nifty, Finnifty) – No circuit
2. Stock futures and options – No circuit (but underlying stock has price band)
3. Currency and commodity derivatives – Specific limits apply

F&O Ban Period

MWPL Calculation

The MWPL is the maximum combined open interest allowed across all F&O contracts of a stock.

Derivation:

The MWPL has two constraints – whichever is lower applies:

Constraint 1: 20% of free-float shares

This prevents derivative positions from becoming too large relative to the actual tradeable stock supply.

$N_{\text{max,shares}} = 0.20 \times N_{\text{free-float}}$

Where $N_{\text{free-float}}$ is the number of shares available for public trading (excluding promoter holdings, strategic holdings, locked-in shares).

Constraint 2: ₹500 crore notional value

$N_{\text{max,value}} = \frac{\text{₹}500\text{ crore}}{P_{\text{current}}} = \frac{5 \times 10^9}{P_{\text{current}}} \text{ shares}$

Where:

• ₹500 crore $= 500 \times 10^7 = 5 \times 10^9$ rupees
• $P_{\text{current}}$ is in rupees per share

Why the ₹500 crore cap? This prevents excessive concentration in high-priced stocks. Without this, a high-priced stock could have enormous notional exposure even with a reasonable share count.

Final MWPL: $\text{MWPL (shares)} = \min(N_{\text{max,shares}}, N_{\text{max,value}})$

The ban trigger occurs when: $\text{Current Open Interest} > 0.95 \times \text{MWPL}$

Step 1: Calculate 20% of free-float constraint $N_{\text{shares}} = 0.20 \times 350\text{ crore} = 70\text{ crore shares}$

Why this step? This is the maximum allowed based on available supply.

Step 2: Calculate ₹500 crore notional constraint $N_{\text{value}} = \frac{5 \times 10^9}{3500} \approx 1.428 \times 10^6 \text{ shares} \approx 0.143\text{ crore shares}$

Why this step? High-priced stocks like TCS are limited by rupee value, not share count.

Step 3: Determine MWPL (take minimum) $\text{MWPL} = \min(70\text{ crore}, 0.143\text{ crore}) = 0.143\text{ crore shares}$

Why the minimum? Both constraints must be satisfied; the stricter one binds.

Step 4: Calculate ban threshold $\text{Ban threshold} = 0.95 \times 0.143\text{ crore} \approx 0.136\text{ crore shares}$

Step 5: Check if ban applies Current OI = 0.5 crore shares > 0.136 crore shares → TCS would be in F&O ban

Why this matters? No new positions allowed until OI falls below the exit threshold.

What the trader CAN do:

1. Square off – Sell 100 futures to close position ✓
2. Let expire – Hold till expiry and settle ✓
3. Exercise (if options) – Exercise ITM options ✓

What the trader CANNOT do:

1. ❌ Open fresh long – Buy additional futures
2. ❌ Open fresh short – Sell new futures
3. ❌ Rollover by increasing – Cannot increase position while rolling to next month
4. ✓ Rollover same size – Can rollover 100 contracts to next month (square off current + open same size in next month = net zero fresh position)

Why these restrictions? The goal is to reduce overall open interest, not maintain it. Allowing fresh positions would defeat the purpose.

Ban Removal Criteria

The stock exits the ban when open interest falls below 80% of MWPL. This creates a buffer zone:

• Enter ban: >95% MWPL
• Exit ban: <80% MWPL

Why the 15% gap (hysteresis)? Prevents flip-flopping. Without this buffer, stocks would enter and exit ban repeatedly as OI hovers near 95%, causing confusion and operational issues.

Comparison: Circuit Filters vs F&O Ban

Common Mistakes

Why it feels right: Seems logical that if spot trading is limited, derivatives should be too.

The fix: Stock derivatives have NO circuit filters. Only the underlying spot stock has price bands. Futures and options continue trading even when the spot stock is at its band, providing price discovery beyond the band.

Example: Reliance spot reaches upper band at ₹3,000. Reliance futures can trade at ₹3,010, ₹3,020, signalling demand beyond the band.

Why it feels right: People confuse individual-stock price bands with market-wide index circuit breakers, which DO halt trading.

The fix: For individual stocks, hitting the band only means orders beyond the limit are rejected — trading continues at/within the band. A cooling-off trading halt applies only to market-wide index circuits (Nifty/Sensex ±10%/15%/20%).

Why it feels right: "Ban" sounds like everything is blocked.

The fix: You can always square off existing positions during ban. The ban only prevents fresh positions. In fact, the exchange wants you to exit to reduce OI. It's like a nightclub at capacity — existing guests can leave anytime, but new guests can't enter.

Why it feels right: "95%" is in the trigger condition, so people mix it with the base calculation.

The fix:

• MWPL = 20% of free-float (not total shares) OR ₹500 crore limit (whichever is lower)
• 95% is the threshold of MWPL that triggers the ban
• These are separate: $\text{Ban trigger} = 0.95 \times \text{MWPL}$

Practical Implications

For Intraday Traders

1. Stock at its band → hard to exit if you're on the wrong side (few counter-parties)
2. Solution: Always use stop-losses tighter than circuit limits
3. Derivatives advantage: Can exit via futures if spot is stuck at its band

For F&O Traders

1. Monitor NSE ban list daily (published on nseindia.com)
2. Rollover strategy: If stock enters ban before expiry, you can rollover (same size) but cannot increase
3. Premium impact: Options premiums can spike when ban is announced (implied volatility surges)

For Risk Management

1. Avoid stocks near 90% MWPL → High ban risk
2. Band breaches → Signal extreme volatility (reassess thesis)

F&O ban is different. Imagine the stadium has limited seats (like a movie theatre). When 95% of seats are filled with people all betting on the same team, the manager says "Hold on! Too many people are betting on this one team. No new people can buy betting tickets, but people already inside can leave anytime." This prevents dangerous overcrowding.

The key difference: one caps how far the price can move (circuit), the other limits new people joining the betting crowd (F&O ban).

Or: "Circuits Cap Price, Bans Block Size"

Connections

• 6.7.1-NSE-BSE-Market-Structure – Exchange architecture enabling these controls
• 6.7.5-Margin-Requirements-SPAN – How margins adjust during high volatility
• 6.7.8-Market-Wide-Circuit-Breakers – Index-level trading halts (the actual cooling-off mechanism)
• 5.4.3-Implied-Volatility-Calculation – IV spikes when stocks enter F&O ban
• 3.2.7-Risk-Management-Position-Sizing – Why monitoring MWPL matters for portfolio risk
• 6.7.14-Settlement-Mechanisms – How positions settle when stuck in ban period

#flashcards/stock-market

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Hinglish (regional understanding)

Hinglish (regional understanding)