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MT4 Guide on limits for accessible external surfaces that may be accidentally touched in normal use.
Introduction
In October of 2010, IEC published IEC Guide 117, Electrotechnical equipment – Temperatures of touchable hot surfaces, for assessing the risk, to any person, of a burn from contact with hot touchable surfaces of electrotechnical equipment. The guide requires that technical committees take into consideration its temperature limits when establishing temperature limits in product standards.
IEC TC 61 has assigned to its MT 4 the task of maintenance of temperature limits in the IEC 60335 series of standards. As such, MT 4 prepares DCs for the parts 2 in the IEC 60335 series to address, if necessary, the application of IEC Guide 117.
This document is for use by MT 4 to aid in the preparation of these DCs.
For MT 4’s purposes, this Guide is for determining the need for and the values for surface temperature limits in appliance standards, taking into consideration:
– the likelihood of contact with the heated part;
– the size and thermal capacity of the heated part;
– the expertise of the persons and their knowledge and experience relative to the temperatures likely to be encountered in operating or servicing the product;
– the provision of adequate cautions or warnings; and
– other similar factors taking into account the specific IEC 60335 Part 2 appliance(s) and its application.
Temperature limits for the following zones or surfaces are not specified:
– hot functional surfaces;
– adjacent surfaces;
– surfaces not likely to be touched.
Table 3 of the standard already addresses surfaces of handles, knobs, grips and similar parts, whether continuously held or held for short periods only in normal use. This Guide does not amend the established normal temperature rises for those parts.
The requirements for hot touchable surface temperatures shall generally be specified as described in the Part 1. However, where the likelihood for user contact with hot touchable surfaces warrants, the user safeguard of temperature limits for those surfaces shall be applied.
To assess the likelihood for user contact with hot touchable surfaces, the following information shall be obtained:
– the touchable surfaces that may be touched unintentionally,
– the users, or other persons, who will likely touch or may touch the surfaces unintentionally,
– the range of operation of the product,
– the probability of touching a touchable surface,
– statistical data on relevant incidents, if available, and
– the normal operation setting of the temperature of the product.
Template for Modification of Part 2 Standards
The MT 4 template for implementation of IEC Guide 117 includes the following:
|Clause |Clause title |Details |
|2 |Normative references |Adding reference to IEC 60584-1. Required when the figure for the probe for measuring |
| | |surface temperatures is included. |
|3 |Terms and definitions |Adding definitions of “functional surface” and “adjacent surface”. |
|7.1, 7.6, 7.12, |Marking and instructions |Adding the hot surface marking and its symbol, explanation of the symbol’s meaning, minimum|
|7.14, 7.15 | |size of the symbol, location of the symbol. |
|11.3 |Heating |Adding the temperature measurement probe. |
|11.8 |Heating |Implementation of the temperature limits from IEC Guide 117. |
|Figures |Figures |New figure for identifying surfaces |
| | |New figure for the temperature measurement probe |
| | |New figure for surfaces that are not measured. |
Implementation
To prepare DCs, the document embedded below provides guidance on the MT 4 standardized modifications.
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The remainder of this document provides rationale and history.
Application of IEC Guide 117 by MT 4
MT 4 has established four categories for products in the IEC 60335 series. These are:
• Category F (floor)
Appliance intended to be used on the floor. A fixed appliance mounted less than 850mm from the floor
• Category W (work surface)
Appliance intended to be used on a work surface, a fixed appliance mounted at or above 850mm and below 2.25m.
• Category C (Commercial)
Commercial appliance intended to be used by a trained person.
Note: Category C limits are the same as that for motor-operated appliances in the part 1 (edition 5.2) table 3.
• Category N (No additional limit)
This category is for appliances where, after consideration of the principles described in the introduction of this document, only the temperature limits in the part 1 apply.
1 Temperature Limits
The temperatures to be considered as basis for the work MT4 shall do on Parts 2 are listed as follows:
The actual values need to consider the risk. See Annex and the limits can vary in the specific Part 2s.
|External enclosure of appliances except handles |Category F |Category W |Category C |
|held in normal use: | | | |
|– of bare metal |38K |42K |48K |
|– of coated metal |42K |49K |59K |
|– of glass and ceramic |51K |56K |65K |
|– of plastic having a thickness exceeding 0,4 mm |58K |62K |74K |
|a, b | | | |
Thermocouple temperature uncertainty is in the region of ±5.3K (from IECEE Guide 001)
In order to cope with the use or interaction with appliances by vulnerable persons at the Melbourne Ad Hoc temp meeting we decided for Category F (the only area to be covered under the terms of reference) to use Guide 117, upper curve, no rounding and 4 s touch time which equates to coping with vulnerable persons i.e. Children over 3 years.
For very vulnerable persons i.e. children under 3 it was decided that parental responsibility is vital.
For Category C the limits should be based on Guide 117 using the 0.5 s touch time limits for healthy adults since the trained individuals are under the supervision of employers etc. who by legislation need to determine whether the individual is competent and could use the appliance in a way that ensured the safety of the operative and the other persons in the vicinity of the appliance which are most likely to be other employees.
The area which was not fully dealt with in Melbourne was the use of household appliances above 850 mm i.e. on a work surface where access by vulnerable adults such as the elderly would be expected as they would use appliances. However when a person has major disabilities and inability to care for themselves (i.e. very vulnerable) the standard would not cover such persons.
Between the ages of 6 and 14 children learn to use appliances and would have access by reach to the work surface. For this group 2 s is recommended by Guide 117 and would also be satisfactory for those adults with non-complex disabilities and those elderly whose reaction times are slowing down. This latter group whilst having slow reaction times also has a lifetime of experience that would help them avoid contact with known hot surfaces. For this reason in Europe 2 s touch time was selected and is a valid proposal for world values. To keep in line with the other decisions of Melbourne the values should be based on upper curve without rounding whereas in Europe they were rounded up.
Commercial appliances should use category C limits since they are intended to be used in a “professional” environment. However when operating in commercial premises but in a public unsupervised area the appliance may need to revert to household limits i.e. Category F or W.
Where the part 2 allows more than a single category of limits manufacturers will be required in 7.12.1 to state the minimum installation height of the fixed appliance so that the correct limits apply otherwise the lowest limits i.e, category F will apply.
area open to the public
area in which the general public including children can enter
2 Notes to the tabled values above
These notes also apply to the limits for knobs and handles and were agreed at the Berlin meeting of IEC TC 61.
a Metal is considered coated when a coating having a minimum thickness of 90µm made by enamel, powder or non-substantially plastic coating is used.
b The temperature rise limit of plastic also applies for plastic material having a metal finish of thickness less than 0,1 mm.
c When the thickness of the plastic coating does not exceed 0,4 mm, the temperature rise limits of the coated metal or of glass and ceramic material apply.
Note: With a 0.4mm coating the value in Category W is actually 64K . i.e. 2k higher than that of plastic.
When deciding whether a surface is inaccessible i.e. undersurtace or rear the use the 75mm diameter probe having a hemispherical end.
3 Allowance for higher limits by using a warning.
Where specific appliances due to construction and or size need higher limits the allowance to use a warning in Clause 7 is permitted with a limit of 2X the values shown in the table above. Only applicable to the Part 2s not to Part 1.
Note: Decision on the use of label to be taken on a product by product basis.
4 Hot surfaces
Functional hot surfaces have no limits this would include actual heating elements e.g. on a hob and accessible vessels such as the heated fluid containing body of a kettle , coffee pot, roaster and frying pans including their lids.
The adjacent surfaces are difficult to define so it is easier to define distances from hot functional surfaces of parts made of similar materials etc.
5 Test parameters in Clause 11
The use of test probe from -2-6 to measure temperatures on flat surfaces should be permitted.
The need for the requirements in 22.13 should be reconsidered in the Part 2s since it might conflict with the changes being proposed by MT 4 in Clause 11.
David Briere, P. Eng.
Convenor, IEC TC 61 MT 4
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61(Shanghai/MT4)43
October 2019
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