Periodic Call Auction on NSE: How Illiquid Securities Trade in Hourly Sessions

Try buying a few hundred shares of a tiny, barely traded company on NSE some afternoon and you may notice something odd. Your order doesn’t fill instantly, even at a sensible price. It just sits there until the top of the hour, when a batch of trades suddenly prints at one price. That’s the periodic call auction, the separate trading route NSE uses for illiquid securities, and if you hold small-caps or micro-caps it’s worth knowing exactly how it works.

We’ll go through who ends up in it, how a session runs minute by minute, how the single clearing price is worked out, and the one rule that can cost a careless trader ₹5,000 in a single session. There’s a worked example with a made-up company at the end of each section so the arithmetic is clear.

What the periodic call auction is and why it exists

In the normal market, NSE runs continuous matching. A buy order meets the best available sell order the moment they cross, trade after trade, all day. That works well when there are plenty of buyers and sellers. In a stock that trades a handful of times a day, it doesn’t. One stray order can swing the price several percent, and the last traded price stops meaning much.

A call auction flips the approach. Orders are collected for a fixed window, nobody trades during that window, and then the exchange matches everything at one price, the price that lets the largest quantity change hands. Pooling orders like this gives thin stocks a fairer shot at price discovery.

SEBI’s framework says it plainly: trading in illiquid scrips in the equity market “shall be conducted only through periodic call auction sessions.” So once a stock is in, it can’t be traded in the continuous session at all. NSE reproduces the full set of rules, including the later changes, on its periodic call auction page, along with the list of circulars going back to 2013.

If you’ve read our explainer on the NSE pre-open session, the mechanism will feel familiar. The pre-open uses a call auction once a day to set the opening price for regular stocks. Illiquid securities get the same treatment, but repeated through the trading day.

Which securities count as illiquid

The test was rationalised in December 2013, and the version NSE now publishes runs in two steps.

Step one: the turnover test. A scrip that trades in the normal market, and hasn’t been shifted to trade-for-trade settlement, is classified as illiquid on an exchange if both of these hold:

• its average daily turnover is less than ₹2 lakh, calculated over the previous two quarters; and
• it is classified as illiquid at all exchanges where it trades.

Step two: the exclusions. A scrip that passes step one is still kept out of the call auction if it meets any one of these conditions:

• average market capitalisation of more than ₹10 crore;
• the company has paid a dividend in at least two of the last three years;
• the company has been profitable in at least two of the last three years, not more than 20% of the promoters’ shareholding is pledged in the latest quarter, and book value is three times or more the face value.

Read that third exclusion carefully. All three parts have to be true together. Profitability alone won’t get a stock out.

Exchanges identify illiquid scrips at the beginning of every quarter and move them into the call auction. A scrip comes back to normal trading once it has spent at least one quarter in the call auction and no longer meets the illiquidity test. For both entry and exit, the market gets two trading days’ notice.

Worked example: does Konkan Polymers go in?

Konkan Polymers is a hypothetical company we’ve made up for this exercise. Over the previous two quarters its average daily turnover was ₹1.6 lakh, and it is classed as illiquid on every exchange where it trades. Step one is met.

Now the exclusions:

• Average market capitalisation is ₹7 crore. That’s not above ₹10 crore, so this exclusion doesn’t apply.
• It paid a dividend in only one of the last three years. Not enough.
• It was profitable in two of the last three years and only 12% of promoter holding is pledged, both fine. But its book value is ₹18 per share against a face value of ₹10, which is 1.8 times. The rule needs three times or more, so this exclusion fails too.

No exclusion applies, so Konkan Polymers moves to the periodic call auction for the quarter. Change one number and the answer flips: if book value were ₹32 against the same ₹10 face value (3.2 times), the third exclusion would be satisfied and the stock would stay in normal trading.

How a periodic call auction session runs

Each session lasts one hour, split three ways under SEBI’s rules:

• 45 minutes for order entry, modification and cancellation;
• 8 minutes for order matching and trade confirmation;
• 7 minutes of buffer to close the session and move to the next one.

The order entry window doesn’t shut on a fixed tick. It closes randomly, system-driven, somewhere between the 44th and 45th minute. That random close is deliberate. Nobody can wait until the exact last second to drop a large order and move the price without anyone having time to react.

SEBI lets each exchange decide how many sessions to run, with a floor of at least two sessions a day and one uniform closing session across exchanges. NSE’s market timings page lists six sessions of one hour each for the call auction illiquid segment, starting at 9:30 am and running until 3:30 pm. So the windows on NSE are 9:30 to 10:30, 10:30 to 11:30, and so on, with the last one from 2:30 to 3:30.

A few other rules shape what happens inside a session:

• Unmatched orders. After the December 2013 change, orders may stay valid through the trading day, and unmatched orders at the end of a session may be carried into the next session. Check with your broker how they handle this, because it decides whether you need to re-enter an order every hour.
• Price band. A maximum price band of 20% applies through the day, and exchanges can tighten it uniformly on surveillance grounds. Our piece on the NSE price band and circuit limit explains how those bands are set.
• Market-wide circuit breaker. If the index-based market-wide circuit breaker triggers during a session, that session is cancelled and all its orders are purged. Call auction trading picks up again at the nearest half hour after the normal market resumes.

In a call auction the real question isn’t “what’s the last price?” It’s “at what single price can the most shares change hands right now?”

How the single price is decided

The matching logic comes from SEBI’s July 2010 circular on call auctions, which the periodic call auction rules adopt for anything they don’t change. The equilibrium price is the price at which the maximum volume is executable. If more than one price gives that same maximum, the exchange picks the one with the smallest order imbalance, meaning the least unmatched quantity in absolute terms. If there’s still a tie, it takes the price closest to the previous day’s close. And if the previous close sits exactly midway between the two tied prices, the previous close itself becomes the equilibrium price.

The circular includes its own tie-break illustration. Prices of 103 and 96 both allow 2,000 shares to trade with the same unmatched quantity. With a previous close of 95, the price is 96. With a previous close of 105, it’s 103. With a previous close of 99.5, the exact midpoint, the price is 99.5.

Worked example: one session in Konkan Polymers

Say these orders are in the book when the 11:30 session’s order window closes. All prices are in rupees and the numbers are hypothetical.

Here’s how to read it. A buyer willing to pay ₹49 will also happily buy at ₹48, so cumulative buy quantity adds up from the top price down. A seller happy at ₹47 will also sell at ₹48, so cumulative sell adds up from the bottom price up. At each price, the shares that can actually trade are the smaller of the two cumulative numbers.

The biggest tradable quantity is 3,000 shares, at ₹48, and no other price comes close. So ₹48 is the equilibrium price. Every matched trade in that session prints at ₹48, so a matched buyer who bid ₹49 still pays ₹48 and a matched seller who offered at ₹46 still receives ₹48. Total value: 3,000 × ₹48 = ₹1,44,000.

At ₹48 there were 3,500 shares wanted and only 3,000 offered, so 500 shares of buy orders are left unmatched. Under the rationalised rules, those may roll into the 12:30 session rather than vanish.

Two things we’d take away from this. First, bidding ₹49 didn’t cost the ₹49 bidder anything extra, since everyone pays the single price. Second, a single large order at the edge of the book can move the equilibrium a lot in a stock this thin. Add 3,000 more sell shares at ₹46 and redo the cumulative columns: the most shares that can now trade is 4,500, at ₹47, so the clearing price drops a full rupee. Thin books are exactly why these stocks were moved here in the first place.

The self-trade penalty

This rule catches people. If the highest buy price entered by a client, on a PAN basis, is equal to or higher than the lowest sell price entered by that same client, and those orders end up trading, the exchange imposes a penalty on those trades. It’s charged to the trading member daily, the member may recover it from the client, and the money goes to the Investor Protection Fund.

For each instance per session, the penalty is the higher of:

• 0.50% of the trade value for the sale plus 0.50% for the buy, which is 1% in total for the client; or
• ₹2,500 for the buy trade plus ₹2,500 for the sell trade, which is ₹5,000 in total.

Take a hypothetical case in our Konkan session. A client has a buy order at ₹48 and, from the same PAN, a sell order at ₹47, and 1,000 shares match at ₹48. Trade value is ₹48,000. One percent of that is ₹480, which is less than ₹5,000, so the penalty is ₹5,000. The 1% leg only takes over once trade value goes past ₹5 lakh, because 1% of ₹5 lakh is exactly ₹5,000.

The practical lesson is simple. If you trade these stocks from more than one account linked to the same PAN, or you’re adjusting a position and leave an old sell order sitting in the book, check both sides before the window closes.

Practical points if you hold an illiquid stock

1. Time your order to the session. An order placed at 10:50 joins the 10:30 session, whose order entry window shuts at random between 11:14 and 11:15, and only then does matching happen. Nothing fills the moment you place it.
2. Use limit orders. With a thin book, a limit price protects you from a matching price you never intended. Remember that you’ll get the single equilibrium price if your order is matched, which may be better than your limit.
3. Watch the quarterly lists. Exchanges review illiquid scrips at the start of each quarter, with two trading days’ notice before moves in or out. A stock that’s been in the call auction for at least a quarter and no longer meets the test goes back to continuous trading.
4. Don’t confuse it with surveillance. Being in the call auction is about turnover, not wrongdoing. Surveillance frameworks such as ASM and GSM are a separate track, covered in our guide to the ASM and GSM surveillance framework. A stock can face both, but one doesn’t imply the other.
5. Expect gaps. Prices can jump between sessions because each one is a fresh auction. Plan exits with that in mind, especially when you’re holding a position that’s large relative to daily turnover.

The periodic call auction won’t make an illiquid stock liquid. What it does is pool scattered orders so that when shares change hands, they do so at a price more people agreed to. If you own such stocks, understanding the hourly rhythm and the matching logic is a small effort that saves you from nasty surprises.

Disclaimer: This article is for educational purposes only and is not investment advice or a recommendation to buy or sell any security. The company and figures in the worked examples are hypothetical. Investing in equities carries risk. Please do your own research or consult a SEBI-registered investment adviser before making investment decisions.