Final DRAFT – 10/21/99
IPC-1331 Voluntary Safety Standard for Electrically Heated
This voluntary standard establishes the minimum requirements for the
design, insafe installation, and operation of electrically heated process
equipment in order to minimize electrical hazards and prevent fires that
may occur in combustible tanks, tank liners and drying equipment. It is
intended to cover both liquid and gas (e.g.i.e air) heaters used in the
manufacture of printed wiring boards (PWBs) and printed wiring assemblies
(PWAs). Minimum requirements are indicated by the use of the terms "must"
or "shall". Optional enhancements or alternate approaches to
the minimum requirements are indicated by the use of the terms "can"
Please note: standard does not purport to address all of the safety issues
associated with its use. It is the responsibility of the users of this
standard to establish appropriate safety and health practices and determine
the applicability of regulatory limitation prior to its use.
2. Reference documents
2.1 This Standard is intended to be compliant with the following
regulations and codes:
2.1.1 29 CFR 1910 Subpart S-Electrical
2.1.2 ANSI C2-1997 National Electrical Safety Code
- NFPA 70 National Electrical Code, Article 427 of National Electric
- NFPA 79 National Fire Prevention Association
- UL 499 Electric Heating Appliances
- CSA C22.2 No. 88-1958 Industrial Heating Equipment
3 General application and use.
- Electric immersion heaters are generally used to heat aqueous or
They are not recommended for use with flammable solutions, and shall
not be used with class 1 or 2 flammable liquids.
- Users must verify with equipment and/or heater manufacturer and
chemical supplier(s) that heater sheath material is compatible with
theuser’s intended solution solution before installation and use.
If user changes chemical composition, user must ensure that heater
sheath material is compatible with the new solution by checking with
equipment and/or heater manufacturer.
- Electric immersion heaters may ignite combustible tanks or liners.
To prevent ignition, heating elements shouldmust be securely mounted
in a manner that prohibits direct contact with the tank or tank liner.
Heating elements must be located at least one inch (25 mm) from the
tank bottom and side walls or minimum distance recommended by manufacturer,
whichever is greater.
- Electric heating elements, used in both liquid and gas heating applications,
shall be equipped with appropriate and adequate supports to prevent
contact with combustible surfaces as a result of heating element deflection/sag
due to use.
- Electric immersion heaters must be protected from physical damage.
They must be shielded from [physical damage]. [They must be shielded
or located to prevent physical damage from contact with other items
entering tanks such as anodes, cathodes, racks, product, concentrated
chemicals, and/or electrically charged components. Protection is generally
accomplished by proper placement and appropriate guarding.
- Heater elements must be allowed to cool before they are removed
from equipment for service or replacement.
- Tanks must not be drained until heater element has cooled.
- Electric heating devices must be built and tested to comply with
a nationally recognized independent testing laboratory specification
such as Underwriters Laboratories (UL), Factory Mutual (FM), and ?
(ETL) , listedfor their intended application.
- Design and Installation Requirements
Electrically heated process equipment shall comply with the following
design and installation requirements:
4.1 Control Circuit Design
- A corrosion-resistant temperature-sensing element, such as a thermocouple,
thermistor, or resistance thermal device (RTD), shall be used in
conjunction with a for the temperature-indicating controller (see
§4.1.2). The use of a thermostat with set point indication is also
permitted. All sensing elements must be compatible with the environment
in which they are used (i.e., bath chemistry, fumes, or vapors,
etc) or be housed in a chemically compatible thermowell.
- A temperature-indicating controller with both sensor break and
short protection that provides an indication of process set point
shall be used. The controller shall allow the power control device
(see §4.1.3) to be switched off when the tank temperature reaches
the set point or when the system is not being used. A controller
that provides a visual display of both process set point and actual
process temperature may be preferred as a control enhancement for
- An appropriately sized power control device, such as a contactor
relay, solid state relay (SSR), or silicone controlled rectifier
(SCR), shall be used for controlling the availability or the amount
of electrical power to the process heater.
- All liquid heater elements shall contain a thermal limit device,
such as a fusible link,n bimetallic thermostat, or other temperature
regulating device, to detect an over-temperature condition in all
installations where the possibility of combustion exists. The device
shall disable power flow to the heating elements in the event of
an over-temperature condition. This device may be non-resettable,
manually reset, or automatically reset. If the device automatically
resets, power flow to the heating elements must be restricted from
automatically resuming through use of manual latching circuit. Please
note that the type of thermal limit device used will depend upon
the technologies employed.
- A temperature-sensing device must be provided to protect electrically
heated process equipment from over-temperature conditions. This
device shall disable power flow to all heating elements in the event
of an over-temperature condition. This device must be unique to
and redundant with the temperature-sensing element specified in
§4.1.1. This device may be a pre-set temperature switch or a temperature
sensing element, such as a thermocouple, thermistor, or resistance
thermal device (RTD) and its attendant hi-limit controller. The
set point of this over-temperature device must be set at a temperature
value that is less than the maximum temperature limit of the equipment
(i.e., when reached, no damage will occur to the chamber, lining,
vessel or any other component contained within).
- If an adjustable device or controller is used, the maximum adjustment
position that is available must be no greater than the equipment’s
maximum temperature limit (i.e., when reached, no damage will occur
to the chamber, lining, vessel or any other component contained
- If exothermic chemistries are present in the process vessel, the
over-temperature device and associated cooling equipment must be
set at a temperature value to ensure that the exothermic action
is controlled. The over-temperature set point for such chemistries
shall be lower than the maximum temperature limit dictated by the
materials of construction of the vessel or chamber used (i.e., when
reached, no damage will occur to the chamber, lining, vessel or
any other component contained within). Please note that the over-temperature
limit must be non-resettable to prevent resetting to an unsafe temperature,
such as a temperature that is above an exothermic temperature.
- An over-temperature controller for the over-temperature sensing
device specified in §4.1.5 shall disable power flow to all heating
elements in the event of an over-temperature condition. This over-temperature
controller may be non-resettable, manually reset, or automatically
reset. If the controller automatically resets, power flow to the
heating elements must be restricted from automatically resuming
by the use of a manual latching circuit. The over-temperature controller
shall activate a visible light and/or audible alarm to signal that
an over-temperature condition exists.
- For liquid tank heaters, a low-level switch must be included in
the electrical control circuit to disable the heater load whenever
the process liquid level drops to no less than one inch above the
heater elements’ hot zone. This switch shall also be equipped with
a visible light and/or audible alarm to indicate that the switch
has been activated. It is recommended that users choose low-voltage,
low liquid sensors to prevent risk of shock.
- For gas (e.g., air) heaters, a low-flow switch must be included
in the electrical control circuit to disable the heater load whenever
the process gas (e.g. air) flow drops to a value just above the
minimum flow required by the heating elements’ manufacturer for
gas (e.g. air) heaters. This switch shall also be equipped with
a visible light and/or audible alarm to indicate that the switch
has been activated.
A separate safety interruptrelaywith a contactorcoil must be included
in the electrical control circuit and be wired in series with the
thermally-used heater elements specified in §4.1.4 and activated
by the temperature-sensing device §4.1.5, the over-temperature controller
specified in §4.1.6, the low liquid level switch specified in §4.1.7,
or the low flow switch specified in §4.1.8. This contactor is separate
from the power control device sepcified in §4.1.3 and will interrupt
power flow to all heating elements in the case of an over-temperature
- An enable switch or process set point must be included in the
electrical control circuit to disable the heating control (not the
safety interrupt controls) should process cooling be utilized in
addition to process heating.
- A safety interlock must be included in the electrical control
circuit and connected to a process exhaust fan if the process generates
flammable fumes that require ventilation. This will prevent generation
of possible flammable and toxic vapors when the exhaust fan fails,
and the buildup of process vapors to a level above 25% of their
lower flammability limit.
- For liquid heaters, Ground Fault Circuit Interrupters must be
employed whenever risk of contact with electrically charged solutions
- Electric heater elements must be equipped with a ground wire of
sufficient size to carry any fault current. The construction of
both heater and ground should be approved by a nationally recognized
- All installation work must be implemented in accordance with the
latest edition of the National Electric Code.
- Control System Installation
4.2.1 For open-top tanks, the low-level switch sensing
element shall be mounted in a way that allows the low-level setting
to be tested without draining the contents of the tank. This will
facilitate frequent testing of the liquid low level switch.
- For conveyorized equipment, the low-level switch shall be mounted
to allow testing of the low-level switch sensing element without
draining the sump level whenever possible.
- The temperature-sensing element for the thermal over-temperature
switch shall be mounted to facilitate testing without draining the
tank or sump contents on liquid heaters.
- Thermostat temperature indicating sensors and hi-limit sensing
devices shall be secured in a location below the minimum liquid
level and above the bottom of the heating element.
- Electrical wiring shall be designed and installed so as to minimize
condensation and facilitate drainage of condensation away from electrical
connections to prevent arching, bridging and corrosion on the electrical
- Testing of all temperature sensing elements, limit controls and
liquid level devices shall be performed by trained personnel on
a periodic basis to ensure that all components are working properly
and to assure the integrity of the safety interrupt circuit. Monthly
testing is recommended. Any failure must be addressed by the user’s
corrective action policy.
- All testing shall be documented. Records including corrective
action shall be retained for a minimum of one year or per the record
retention policy of the user until at least the next test cycle
- Any replacement of a temperature-controlled circuit element should
be calibrated and proper operation verified prior to placing into
- Calibration of the temperature controller specified in §4.1.2
should be performed per the manufacturer’s recommendation, but no
less than semiannually.
- Calibration of the over-temperature controller in §4.1.6 should
be performed as recommended by the manufacturer, but no less thanon
- Heating System Maintenance
- All heater elements should be inspected and cleaned frequently
to remove any deposits, which may inhibit heat transfer. It is recommended
that inspection and cleaning occur on a monthly basis or more frequently
if used in additive operations.
- All electrical power must be turned off and proper personal protection
equipment worn prior to removing and inspecting heating elements.
- All electrical service to heating elements must be locked out
during idle periods of more than one eight-hour shift.
- The interchanging of parts, which were not included in the original
heating system design, shall be limited unless advised by the equipment/heating
manufacturer. Alternate parts can lead to failures resulting in
either fire or physical injury.