Planning Time Fences and Plan Stability

SEF Reference Guide G27 | Manufacturing OpEx Maturity Assessment

Supports criteria: SO-4 Plan Stability (Planning Fences)

Glossary terms covered: Planning fence, Frozen zone, Slushy zone, Liquid zone, Plan attainment, Cumulative lead time, Cost of plan changes

How to use this guide. This is SEF's standard method and the numbers we start from. Every operation is different, so treat the targets, timings, weightings, and templates here as a starting point and adjust them to your products, customers, and risk. Write your version down before you measure against it. Do not apply a tool just to apply a tool. Every method here carries a cost in time, attention, and discipline, and the benefit has to justify that cost in your operation. If it does not, leave it out and say why. A lower score on a criterion you have deliberately chosen not to pursue is an honest score, not a failure. One thing should not change: the maturity level definitions, because they are the scoring basis for the assessment.

What it is

A planning time fence is an agreed point in time inside which the plan does not change without a deliberate decision. Inside the fence, the plan is locked: people, machines, and material have been committed to it. Outside the fence, the plan can still move as orders and forecasts change.

Every business has fences whether it writes them down or not. A supplier who needs three weeks to deliver a part has already set a material fence of three weeks. A welder who takes eight weeks to hire has set a capacity fence of eight weeks. The question is whether the business knows where its fences are and plans inside them, or keeps changing the schedule inside them and pays for it.

Most planning systems use three zones:

Frozen: the near term. The plan is locked. Changes need a named approver and are logged with a reason.
Slushy: the middle term. Changes are allowed within agreed limits, for example plus or minus 10% by family, or swapping items within a family that use the same material and capacity.
Liquid: the far term. The plan is open and follows the forecast and S&OP decisions.

Fences are not only for production. Demand, production, material, and capacity each have their own fence, and they nest: the longer the lead time to change something, the earlier it has to be decided.

Why it matters

Customers: a plan that holds is a promise that holds. Every change inside the fence pushes some other order late. OTD (G22) shows the result; the fence is where it is won or lost.
Cost: breaking the plan is expensive and usually invisible. Expediting, premium freight, overtime, extra changeovers, and material bought for jobs that were then bumped are real costs that rarely appear as one number.
Capacity: a plan that changes daily needs more flex capacity (cross-trained people, spare machine time, overtime on call) to absorb the churn. The Resource Calculator (G26) shows how much.
People: constant replanning burns supervisors’ and planners’ time and teaches the floor that the schedule does not mean much.
Improvement: level loading (G13), kanban, and standard work all need a stable near-term plan to work.

Key terms

Term	Meaning
Planning fence	The point in time inside which the plan is not changed without a deliberate, approved decision. Also called a time fence.
Frozen zone	From today to the frozen fence. The plan is locked.
Slushy zone	Between the frozen fence and the planning fence. Limited changes allowed by agreed rules.
Liquid zone	Beyond the planning fence. The plan is open.
Demand fence	The point after which customer order changes are no longer accepted into the plan without approval.
Material fence	The point inside which material for the plan is already ordered or committed. Set by supplier lead times and agreements.
Capacity fence	The time needed to change capacity: hours to move people between areas, days for overtime, weeks or months for a shift, a hire, or a machine.
Cumulative lead time	The longest path through purchasing and production for an item, from ordering the first material to finishing the product. Material not held in stock cannot be changed inside this time.
Firm window	The period of a supplier forecast that is a commitment to buy.
Forecast window	The period of a supplier forecast shared for planning only, with any agreed liability for raw material or capacity.
Plan attainment	The share of the frozen plan that was built as planned (right item, right quantity, right day).
Change inside the fence	Any change to the frozen plan: an added or removed order, a quantity change, or a resequence.
Cost of plan changes	The money spent because the plan changed: expediting, premium freight, overtime, extra changeovers, and material bought and not used as planned. Lost sales are excluded; late orders are tracked through OTD (G22).

How to do it

Find the fences you already have. For your main product families, list the cumulative lead time of material not held in stock, the time to move people between areas, the notice needed for overtime, and the time to add a shift or a hire. These set the minimum length of each fence.
Pick a frozen fence you can keep today. Start short and honest. For many operations that is tomorrow’s plan, locked at a set time today (for example 2:00 pm). If that cannot hold, start with the next shift. A short fence that holds beats a long one that is ignored.
Write the fence policy. One page: where each fence sits, what may change in each zone, who can approve a change inside the frozen zone (one named person, usually the plant manager), and what must be logged. Sales, planning, production, and purchasing all sign it.
Set the material fence with purchasing. For each family, check that material for the frozen period is on hand or firmly due. Where cumulative lead time is longer than you can freeze, decide whether to hold a buffer of raw material or common components (G14) so the plan the customer feels can be shorter than the supply lead time. A buffer is a trade: it buys flexibility with inventory, which raises net working capital and carrying cost. Size it to the lead time gap it covers and take it to S&OP for approval (G20), so the business chooses the working capital it spends, rather than buffering everything.
Agree supplier windows. For key suppliers, share a rolling forecast with a firm window and a forecast window, and agree the liability in each (for example, firm for 2 weeks, raw material liability for weeks 3 to 6, planning only beyond). Write it into the supply agreement and send it every S&OP cycle (G20).
Match capacity decisions to their fence. Move people between areas in the daily meeting. Approve overtime with the notice it needs. Decide shifts, hires, and equipment only in the monthly S&OP, because they take longer than any production fence.
Log every change inside the fence. Date, order, what changed, who approved, and the reason (customer request, material, quality, breakdown, sales override, planning error). Keep it simple: a shared log, filled in when the change is approved.
Measure weekly. Changes inside the fence (count), plan attainment (%), and the cost of plan changes (expedites, premium freight, overtime, extra changeovers, material not used as planned). Review them at the weekly production meeting and in S&OP.
Fix the top reasons. Pareto the change log monthly. The usual causes are material shortages (G14), breakdowns (G02), order entry and expedite requests (G19), and forecast misses (G20). Each one has its own guide.
Earn a longer fence. When changes inside the fence stay low and plan attainment stays high for a month or two, extend the frozen fence: one day, then two or three, then a week. Extend the slushy zone and the supplier firm window with it.

What each maturity level looks like

SO-4 Plan Stability (Planning Fences)

Level	Assessment wording	What it looks like on the floor	How to move up
1	No planning fence exists. The schedule changes whenever a customer, salesperson, or shortage demands it.	The schedule is rewritten several times a day. Hot lists drive the floor. Supervisors spend the morning finding out what changed. Nobody can say what tomorrow’s plan is.	Agree a frozen period you can keep, even if it is only the next day, and write it down.
2	A frozen period is defined and written down (at least the next day), but it is broken often and changes are not tracked.	There is a published plan for tomorrow, but it changes most days and nobody knows how often or why. Sales and production argue about changes after the fact.	Name one approver for changes inside the fence, start a change log with reasons, and measure plan attainment weekly.
3	As Level 2, plus changes inside the fence need a named approver and are logged with a reason, and plan attainment is measured weekly.	Changes go through one person. The log shows how many changes happen and why. Plan attainment is posted. The top reasons are known.	Extend the fences to material and capacity using lead times, agree supplier windows, and start measuring the cost of plan changes.
4	As Level 3, plus fences cover production, material, and capacity, are set from lead times, key suppliers work to agreed firm and forecast windows, and the cost of plan changes (expedites, premium freight, overtime, extra changeovers) is measured and reviewed in S&OP.	Purchasing and production work to the same fences. Key suppliers receive a firm and a forecast window every month. The S&OP pack shows the cost of plan changes alongside OTD.	Attack the top reasons for change, extend the frozen fence as stability allows, and trend the measures for 12 months.
5	As Level 4, plus changes inside the fence are rare and trending down, plan attainment has been stable or improving for 12 months, and the cost of plan changes is falling.	The frozen plan is built as planned almost every day. Expedites and premium freight are rare events with a known cause. The fence has been extended at least once because the operation earned it.	Sustain: review fences every S&OP cycle, extend supplier windows, and use the stability to level the load (G13).

Evidence to look for

Document

Written fence policy: fence lengths, rules for each zone, named approver, signed by sales, planning, production, and purchasing.
Change log for the last 3 months with reasons.
Plan attainment trend.
Supplier agreements or forecast letters showing firm and forecast windows.
Cost of plan changes in the S&OP pack (expedite and premium freight spend, overtime, extra changeovers).

Observation

Compare the plan published yesterday with what the floor is running today. How many differences?
Is there a hot list or expedite board? How long is it?
Are there parts staged for jobs that were moved out? Material in receiving with no current order?

Statement (questions to ask and who to ask)

Supervisor: “How often does tomorrow’s plan change after you get it? Who changes it?”
Planner: “Where are your fences? What happens when sales asks for a change inside them?”
Buyer: “Do suppliers know which part of your forecast is firm? What do you spend on premium freight?”
Sales: “What do you tell a customer who wants an order moved up into this week?”
Plant manager: “What did plan changes cost last month?”

Management report

Changes inside the frozen fence per week, by reason.
Plan attainment (%), weekly, 12-month trend.
Cost of plan changes, monthly: expedites, premium freight, overtime, extra changeovers, material not used as planned.
OTD (G22) alongside, to show the link.

Common pitfalls

A long fence on paper. A one-week freeze that breaks every day teaches everyone the fence is fiction. Start short and keep it.
Production-only fences. Freezing the build plan without checking material just moves the chaos to purchasing.
No single approver. If anyone can approve a change inside the fence, everyone will.
Logging without acting. A change log nobody reviews is paperwork. Pareto the reasons and fix the top one.
Counting the wrong cost. Keep the cost of plan changes to measurable spend: expediting, premium freight, overtime, extra changeovers, material not used as planned. Lost sales are too subjective to count here and are usually a lead time issue. Late orders belong in OTD (G22).
Using the fence to say no. The fence is a decision point, not a wall. A change inside it can be the right call for an important customer; the point is that someone decides it knowingly, and the cost is visible.
Buffering instead of stabilizing. Inventory can let a short fence feel longer, but it costs working capital and carrying cost. A buffer bought to cover a plan that keeps changing pays for the instability instead of fixing it.
Suppliers left out. A frozen plan with no firm window to suppliers leaves them guessing, and their lead times grow to protect themselves.

Template

Fence policy (one page)

Item	Our rule
Frozen fence (production)	Example: tomorrow’s plan, locked at 2:00 pm today
Slushy zone and allowed changes	Example: rest of this week; quantity within plus or minus 10% by family; swaps within a family
Liquid zone	Example: beyond this week; follows the S&OP plan
Material fence	Example: material for the frozen period on hand or confirmed due
Supplier windows	Example: firm 2 weeks, raw material liability weeks 3 to 6, forecast only beyond
Capacity decisions	People moves: daily meeting. Overtime: 2 days notice. Shifts, hires, equipment: monthly S&OP
Approver for changes inside the frozen fence	Name
What must be logged	Date, order, change, reason, approver
Measures and review	Changes inside the fence, plan attainment, cost of plan changes; weekly production meeting and monthly S&OP
Signed by	Sales, planning, production, purchasing

Change log

Date	Order or item	Change (add, remove, quantity, resequence)	Reason	Requested by	Approved by	Extra cost (expedite, freight, overtime, changeover)

Weekly measures

Week	Planned orders in frozen period	Changes inside fence	Plan attainment (%)	Cost of plan changes	Top reason

First 30 days

Week 1: List cumulative lead times for the main families and the time to change capacity. Agree a frozen fence you can keep today and name the approver.
Week 2: Write and sign the one-page fence policy. Start the change log. Start measuring plan attainment.
Week 3: Check material against the frozen plan every day. Start tracking expedites, premium freight, and overtime caused by changes.
Week 4: Pareto the change log. Pick the top reason and open a problem-solving effort on it (G05). Report the first month of measures in S&OP.