PCB Design Safety Regulations: Markings, Shock, and Fuse Requirements

This appendix covers PCB safety regulation requirements: hazard categories, safety markings, electric shock and energy hazards, and fuse marking. Formal safety testing or certification should still follow the applicable standards.

Highlights:

• PCB safety markings must use the accepted installation-country language and stay legible after water and gasoline rub tests.
• ELV, primary, and hazardous-energy secondary circuits need prominent electric shock and energy hazard markings.
• Fuse holders require rated current, fusing characteristic, breaking capacity, and, if needed, rated voltage markings.

Part 5: Appendix

1 Safety Considerations in PCB Design

1.1 Overview

The following hazards can cause harm and injury:

• Electric shock
• Energy-related hazards
• Fire
• Thermal hazards
• Mechanical hazards
• Radiation
• Chemical hazards

As a key system part, the PCB is closely tied to all of them.

Standards contain many clauses on safety markings and instructions. Only clauses linked to PCB design, debugging, installation, and maintenance are listed here. Formal testing or certification should still use the formal standards.

Safety Markings

General Requirements for Safety Markings

PCB safety markings should comply with general safety marking requirements.

Use the language accepted in the country where the equipment containing the PCB is installed.

Markings should be durable and conspicuous; consider normal use effects on durability.

Compliance is checked by inspection and a marking test: rub the marking by hand for 15 s with a water-moistened cotton cloth, then for 15 s with a gasoline-moistened cotton cloth. Afterwards, the marking must remain legible, the nameplate must not be easily removable, and edges must not curl.

Electric Shock and Energy Hazards

Component pins are exposed on the PCB surface, so electric shock or energy hazards are highly possible.

For any PCB location with electric shock or energy hazards, such as ELV, primary, and secondary circuits at safety voltage but hazardous energy levels, add prominent markings. Such locations may be inside or outside the operator-accessible area. The marking should state that the hazardous energy level has been exceeded and include a hazard warning symbol.

Fuses on PCBs

Mark on or near each fuse holder the rated current, fusing characteristic (slow, fast, time lag), and breaking capacity (low-breaking, high-breaking). If the holder accepts different voltage ratings, also mark the rated voltage.

Example: T2.5AL, 250V; F3.15AH, 250V

Add six complete markings near the fuse: designation, fusing characteristic, rated current, breaking capacity, rated voltage, and an English warning, for example: F1 F10AH, 250VAC, "CAUTION: For Continued Protection Against Risk of Fire, Replace Only With Same Type and Rating of Fuse."

Replaceable Batteries

When a product uses a replaceable battery, and replacing that battery with an incorrect type can cause an explosion, the equipment must carry a specific warning. Certain lithium battery types are the usual example: a wrong replacement can produce a hazardous failure, so the risk must be communicated at the battery location and in the product documentation. The requirement covers the enclosure, the operating instructions, and the maintenance instructions, because different people may access the battery in different service situations. The warning is not a general safety note; it is tied to the battery replacement action and must be available where that action takes place.

The compliance path depends on where the battery is installed. If the battery is inside an operator access area, the equipment must either carry a marking adjacent to the battery or state the warning in both the operating instructions and the maintenance instructions. The first option places the warning directly at the point where the battery is replaced. The second option relies on the documentation set: the operating instructions address the operator, while the maintenance instructions address service personnel. Either path is acceptable, but the chosen path must be complete. A label alone is sufficient under the first option, and the two manuals together are sufficient under the second option; one manual alone is not.

If the battery is installed elsewhere in the equipment, outside the operator access area, the requirement changes. In this case, the equipment must either carry a marking adjacent to the battery or state the warning in the maintenance instructions. The operating instructions are not part of this path because the operator does not normally reach the battery. The maintenance instructions become the primary document for the person who opens the equipment and replaces the battery. A marking next to the battery remains an alternative, and it is often used where service access is infrequent or where the label can be seen during replacement. In both location cases, the purpose is the same: the warning must reach the person who is about to replace the battery, whether that person is an operator or a service technician.

The marking or instruction must include the following statement or wording with the same meaning:

CAUTION
Replacing the battery with an incorrect type creates an explosion hazard. Used batteries must be disposed of according to the instructions.

The wording has two functions. It warns that an incorrect battery type can cause an explosion, and it tells the user or service person what to do with a used battery. Both points should be retained when the text is adapted for a manual or a label. A warning that mentions only the explosion risk is incomplete, because it does not address disposal. A warning that mentions only disposal is also incomplete, because it does not explain the replacement hazard. The allowance for similar wording permits translation and layout changes, but the technical meaning must remain unchanged. The word CAUTION should be retained as the signal word when the label or manual uses this format.

When the warning is placed in a manual, it should be located where the battery replacement procedure is described. When the warning is placed on the equipment, it should be adjacent to the battery so that it is visible before or during replacement. The label should not be hidden behind a cover that must be removed before the warning can be read, because the warning is most useful at the moment the battery is exposed. The same logic applies to the manual: the warning should be close to the replacement instructions, not buried in a general safety section without a cross-reference.

General Warning Label Reference List

Warning labels are also selected by item code and size. The reference list below, dated 2000.02.15, associates each label type with its graphic, item code, size specification, and drawing number. The list covers electric shock, microwave, optical radiation, anti-static, grounding, functional grounding, protective grounding, and tamper-evident labels. Several label types are available in more than one size, so the same warning symbol can be matched to different enclosure areas and space limits. The item code identifies the label in the parts system, and the drawing number identifies the controlled artwork and dimensions.

The size specification is listed separately because the same warning can be produced in different dimensions. For most categories in the list, three sizes are given. The 8mm × 8mm size appears in several categories and is the smallest format in those groups. The anti-static label uses 16mm × 18mm, 12mm × 13mm, and 8mm × 8mm, while the optical radiation label includes a 10mm × 10mm size in addition to 16mm × 16mm and 8mm × 8mm. The protective grounding label is listed at 16mm × 16mm, and the tamper-evident label is listed at 50mm × 12mm.

The electric shock, microwave, and optical radiation labels are hazard warning labels. The anti-static, grounding, functional grounding, and protective grounding labels identify ESD and grounding conditions. The tamper-evident label identifies a condition that changes when the equipment is opened. These categories are kept separate because the symbol and the meaning must match the hazard or condition at the marked location. A label from one category should not be used to satisfy a requirement from another category, even if the sizes are the same.

Because the table combines symbol, part number, size, and drawing control, a label should not be replaced by a visually similar symbol without checking the item code and drawing number. The label type must match the hazard, and the size must fit the available area on the equipment. This is particularly important for labels placed adjacent to a battery, because the warning must remain legible and must not be confused with a different safety symbol. The same control applies when a label is added to a manual as a graphic: the correct symbol and the correct wording should be used together. The reference list supports label selection; it does not replace the location rules for the replaceable battery warning described above.

Creepage Distance and Clearance

Every trace, pad, and copper feature on a PCB must satisfy the applicable requirements for clearance and creepage distance. Two reference tables define the minimum values. Table 1 applies to power supplies with a 150V–300V input, and Table 2 applies to power supplies with a 300V–600V input. Both tables are organized the same way: the left half covers the primary side, the right half covers the secondary side, and each half is split into the spacing from the line conductor to protective earth plus the air gap and creepage distance demanded by a given working voltage.

Clearance is the shortest path through air between two conductive features, while creepage distance is the shortest path measured along the surface of the insulating material between those same two features. The two are tabulated as separate columns because they are limited by different breakdown mechanisms and do not scale identically with voltage. Reading down either table, the creepage distance increases steadily as the working voltage rises, whereas the air gap moves in coarser steps and is in some rows left blank. Where the air gap cell is empty, only the creepage distance is called out for that working voltage, so the creepage figure becomes the dimension that governs the row. Observing these insulation-spacing requirements is also an important part of EMC design, helping ensure electrical safety and reliable operation under the specified working conditions.

Table 1: Minimum Clearance and Creepage Distance for a 150V–300V Input Supply

Interpreting the Primary Side of Table 1

The primary side of Table 1 fixes the spacing between the line conductor and protective earth at 4.0 mm. That figure is set once for the whole table and does not vary with the working voltage row, so it must be maintained regardless of which voltage entry applies to the circuit being laid out. Beside it, the working voltage is listed as a DC value or an RMS value, and for each entry the table gives an air gap and a creepage distance.

At 50V, the primary side requires a 1.0 mm air gap and 1.2 mm of creepage. At 150V, the air gap rises to 1.4 mm and the creepage distance to 1.6 mm. The 200V and 250V rows quote only a creepage distance, 2.0 mm and 2.5 mm respectively, with no separate air gap value called out. At 300V, the air gap is 1.7 mm and the creepage distance is 3.2 mm. The table then extends past the nominal input range to cover higher working voltages that may appear inside the circuit: 400V requires 4.0 mm of creepage, and 600V requires an air gap of 3.0 mm together with 6.3 mm of creepage.

Interpreting the Secondary Side of Table 1

The secondary side of Table 1 is indexed by a different set of working voltages and calls for a line-to-protective-earth spacing of 2.0 mm, half the value required on the primary side. The air gap column stays at 0.7 mm across every voltage listed, from 71V through 250V, while the creepage distance steps upward with voltage: 1.2 mm at 71V, 1.5 mm at 125V, 1.6 mm at 150V, 2.0 mm at 200V, and 2.5 mm at 250V. Because the air gap is constant while the creepage distance grows, the creepage requirement is clearly the more demanding one on the secondary side for this input range, and it becomes progressively more dominant as the working voltage increases.

Table 2: Minimum Clearance and Creepage Distance for a 300V–600V Input Supply

Interpreting the Primary Side of Table 2

For the wider input range, the primary-side line-to-protective-earth spacing increases to 6.3 mm. The lower working-voltage rows are sparser than in Table 1: at 50V and 150V only the creepage distance is given, 1.2 mm and 1.6 mm, with the air gap left unlisted. From 200V upward both columns are populated: 2.0 mm air gap and 2.0 mm creepage at 200V, 2.0 mm and 2.5 mm at 250V, 2.5 mm and 3.2 mm at 300V, 3.5 mm and 4.0 mm at 400V, and 5.8 mm and 6.3 mm at 600V. Note that the air gap only exceeds the creepage distance at the top of the range, at 600V, where 5.8 mm of air clearance is required against 6.3 mm of surface distance.

Interpreting the Secondary Side of Table 2

The secondary side of Table 2 uses a line-to-protective-earth spacing of 2.5 mm, slightly above the 2.0 mm of Table 1. The 71V and 125V rows give creepage distances only, 1.2 mm and 1.5 mm. From 150V upward, the air gap is 1.7 mm for every listed voltage, while the creepage distance again climbs with voltage: 1.6 mm at 150V, 2.0 mm at 200V, and 2.5 mm at 250V. At 150V, the air gap of 1.7 mm is actually larger than the creepage distance of 1.6 mm, so in that row the clearance requirement is the one that constrains the layout rather than the surface distance.

Comparing the Two Tables

A higher input range does not simply scale every entry upward by a common factor, and comparing the tables row by row makes that clear. On the primary side, the line-to-earth spacing moves from 4.0 mm to 6.3 mm, and on the secondary side from 2.0 mm to 2.5 mm. Across the primary side, the creepage distance quoted for a given working voltage is identical in both tables: 1.2 mm at 50V, 1.6 mm at 150V, 2.0 mm at 200V, 2.5 mm at 250V, 3.2 mm at 300V, 4.0 mm at 400V, and 6.3 mm at 600V. What changes is the air gap and the line-to-earth spacing. At 300V, for example, the creepage distance is 3.2 mm in both tables, but the air gap grows from 1.7 mm in Table 1 to 2.5 mm in Table 2. At 200V and 250V, Table 1 lists no air gap at all, whereas Table 2 requires 2.0 mm.

On the secondary side, every working voltage that appears in both tables carries the same creepage distance, while the air gap differs sharply: 0.7 mm in Table 1 against 1.7 mm in Table 2 at 150V, 200V, and 250V. The practical consequence is that the higher input range mainly widens the air clearance and the distance to protective earth, and leaves the surface-distance requirement unchanged for a given working voltage.

Applying the Tables During Layout

The tables are indexed by working voltage, not by the nominal input voltage of the product, so the first step is to establish the input range of the supply and choose the matching table: Table 1 for a 150V–300V input, Table 2 for a 300V–600V input. The second step is to identify the working voltage, expressed as a DC value or an RMS value, that appears across each gap that needs to be checked. That figure selects the row; the input voltage only selects the table.

With the row selected, three checks follow. The line-to-protective-earth spacing from the outer column of the relevant side must be met wherever a line conductor runs near the protective earth. The creepage distance must be met along the surface of the board between conductors at the working voltage in question. The air gap must be met through any opening, cutout, or air path between those same conductors. Where both an air gap and a creepage distance are listed for a row, the layout must satisfy both; where the two differ, the larger of the two controls the geometry at that location. The 150V secondary row of Table 2, with its 1.7 mm air gap against 1.6 mm of creepage, and the 50V primary row of Table 1, with its 1.2 mm of creepage against a 1.0 mm air gap, both illustrate how the controlling dimension can switch from one column to the other depending on the row.

Because the tables list discrete working voltages, each gap in the design should be matched to the row that covers its working voltage, and the resulting minimum should be treated as a floor that the layout must not fall below. Building the check into the layout review, at every point where two conductors at different potentials face each other along the primary side, along the secondary side, and to protective earth, is the reliable way to keep the finished board compliant with these two tables.

Coated Printed Circuit Boards

The following aspects of coated PCBs depend on PCB material properties and manufacturer quality control. Hardware and CAD engineers cannot control them directly, but they are important safety items in PCB design. Recommended values are given for reference.

Mechanical Strength of the PCB

The PCB should have adequate mechanical strength. Inserting and removing a board applies stress to both the board and the motherboard. When the board or motherboard is large, this stress can lead to a range of unknown performance and safety problems.

No current standard sets mechanical strength limits for boards or motherboards under such stress, and no empirical values have been established. Further study is needed.

Flame Retardance Rating of PCB Materials

The PCB should have an appropriate flame retardance rating. The recommended rating is V-2.

Thermal Cycling and Thermal Aging Tests

A coated PCB should pass the thermal cycling and thermal aging tests specified in IEC60950.

Electric Strength Test

A coated PCB should pass the electric strength test specified in IEC60950.

Scratch Resistance Test

A coated PCB should pass the scratch resistance test specified in IEC60950.

Routing and Power Supply

Power traces on the PCB should have sufficient current-carrying capacity. The table below gives the relationship between trace width and current-carrying capacity. Insufficient capacity can overheat the board and create a range of safety problems.

Trace Carrying Capacity per MIL-STD-275

Current-carrying capacity is a basic constraint in printed wiring layout. A trace must carry its intended current without exceeding the temperature rise allowed for the board, and the maximum current depends on the trace width, the copper weight, and the temperature rise that the design can accept. Table 3 provides maximum current values for printed wiring across a set of trace widths, copper weights, and temperature-rise categories. The table is identified as trace carrying capacity per MIL-STD-275, and the accompanying note cites MIL-STD-275, Printed Wiring for Electronic Equipment. Trace width is expressed in inches, and the tabulated current values are maximum current in amps.

Table 3: Relationship Between Trace Width and Current-Carrying Capacity

Reading the Table by Width, Copper Weight, and Temperature Rise

Each row in Table 3 corresponds to a trace width, beginning at .010 inch and extending through .250 inch. The current columns are grouped by temperature rise: 10 C, 20 C, and 30 C. Within each temperature-rise group, the columns give the 1/2 oz., 1 oz., and 2 oz. copper weights. To read the table, first locate the trace width in the leftmost column. Then select the temperature-rise group that matches the design allowance. Then select the copper weight. The cell at that intersection is the maximum current in amps for that combination.

Because the table repeats the copper-weight sequence under each temperature-rise heading, it can be used for direct comparison. For a given width and copper weight, moving from the 10 C group to the 20 C group and then to the 30 C group shows how the tabulated maximum current changes with the permitted temperature rise. For a given width and temperature rise, moving across the three copper-weight columns shows the effect of copper thickness. For a given temperature rise and copper weight, moving down the rows shows the effect of increasing trace width.

Trace Width Rows and Copper-Weight Columns

The first column lists the trace widths used in the table: .010, .015, .020, .025, .030, .050, .075, .100, .200, and .250 inch. These are the width values for which maximum current is tabulated. The current columns are grouped by temperature rise. Under each temperature-rise heading, the columns repeat the copper-weight sequence of 1/2 oz., 1 oz., and 2 oz. The temperature-rise headings are 10 C, 20 C, and 30 C. This arrangement lets the reader keep the trace width fixed while comparing copper weights within one temperature-rise group, or compare temperature-rise groups while keeping the copper weight fixed.

The copper-weight headings use the notations 1/2 oz. (OZ.), 1 oz. (oz.), and 2 oz. (Oz.). The table therefore distinguishes the three copper weights directly in the column labels. The temperature-rise headings are shown as 10 C, 20 C, and 30 C. The current entries are given in amps. The trace-width entries are given in inches, as stated in the note on Table 3.

Using the Table for a Current Check

For a design check, the table can be read from the required current and the allowable temperature rise toward a suitable trace width and copper weight. First identify the temperature-rise group that matches the design allowance. Then select the copper weight that will be used in the printed wiring. Within that group and column, read down the rows until the tabulated maximum current is at least the required current. The corresponding trace width is the width listed for that row. This inverse reading follows directly from the table layout: width is the row variable, while temperature rise and copper weight are the column variables.

The table gives discrete entries. It does not list continuous curves or interpolation values; each maximum-current value belongs to a specific width, copper weight, and temperature-rise combination. The reader should therefore use the entry for the exact combination shown. When the table is used for comparison, the width should be held constant while the temperature-rise group and copper weight are varied, or the temperature-rise group and copper weight should be held constant while the width is varied. Keeping one variable fixed makes the effect of the other variables easier to see.

Observed Relationships in the Tabulated Values

The table shows the general dependence of current capacity on trace width, copper weight, and temperature rise. The effect of copper weight is visible in the .010 inch row. At a 10 C temperature rise, the table lists .5 A for 1/2 oz., 1.0 A for 1 oz., and 1.4 A for 2 oz. At 20 C, the same width lists .6 A, 1.2 A, and 1.6 A. At 30 C, it lists .7 A, 1.5 A, and 2.2 A. For this width, the heavier copper columns carry higher maximum current within each temperature-rise group.

The effect of trace width is visible by comparing rows within one temperature-rise and copper-weight column. In the 10 C, 1/2 oz. column, the table gives .5 A at .010 inch, 1.5 A at .050 inch, 2.6 A at .100 inch, and 5.0 A at .250 inch. The wider rows carry higher maximum current in that column. Similar comparisons can be made in the other columns by reading down the table while keeping the temperature-rise heading and copper-weight heading fixed.

The effect of temperature rise is visible by comparing the three temperature-rise groups for a fixed width and copper weight. At .050 inch and 1/2 oz., the table lists 1.5 A at 10 C, 2.0 A at 20 C, and 2.6 A at 30 C. At .100 inch and 1 oz., it lists 4.2 A at 10 C, 6.0 A at 20 C, and 7.5 A at 30 C. These entries show the tabulated maximum current increasing as the permitted temperature rise increases. The table is arranged so that these comparisons can be made directly from the columns, without changing the trace-width row.

The values in the .200 inch row also illustrate why the table should be read cell by cell rather than treated as a smooth formula. At 10 C, the 2 oz. column lists 11.5 A; at 20 C, the 2 oz. column lists 11.0 A; at 30 C, the 2 oz. column lists 20.5 A. The table reports the values for each listed combination, and the reader should use the entry that matches the intended temperature rise and copper weight. The same principle applies throughout the table: the row, the temperature-rise group, and the copper-weight column together define the tabulated current.

Notes on Table 3

Table 3 is captioned as the relationship between trace width and current-carrying capacity. The note on the table identifies the reference as MIL-STD-275 Printed Wiring for Electronic Equipment. The trace-width unit is inch. The table's current entries are maximum current in amps. The copper weights are 1/2 oz., 1 oz., and 2 oz., and the temperature-rise categories are 10 C, 20 C, and 30 C. The table does not introduce a separate width unit for the current columns; the width unit applies to the first column, and the current unit applies to the tabulated values.

The table is a tabulated reference for the listed combinations. It gives the maximum current for each width, copper weight, and temperature rise shown. When the required current, allowable temperature rise, copper weight, and trace width correspond to one of the table entries, the maximum current can be read directly from the intersection. When comparing candidate designs, the reader can move across a row to compare copper weights under the same temperature rise, or move down a column to compare widths under the same copper weight and temperature rise. These are the comparisons the table is structured to support.

Related Reading:

• Capacitor Selection and Placement for PCB EMC Design
• Transmission Line Models: Microstrip, Stripline, and Reflections
• Surface Microstrip vs. Inner-Layer Stripline Routing
• Impedance Control Basics: Characteristic Impedance Explained
• Special Signal Handling and Via Effects in PCB Design
• Backplane Slot Layout and Board Position Structure for EMC Design
• Backplane Connector, Impedance, and Power/Ground Design for EMC
• PCB EMC Design: Module Partitioning and Special Component Layout