EN 953 to be replaced by EN 14120 | Guarding Standard

Sep 17, 2015

There is a new standard on the way. It updates the guidance on design and selection of guards for machinery and it is going to change the way you design and adapt guarding on your machines in the future.

Draft standard ISO 14120 Safety of machinery — Guards — General requirements for the design and construction of fixed and movable guards

This new guarding standard is expected to be published early next year. Once published, it is expected to be harmonised as a European standard EN ISO 14120; replacing EN 953 in the Official Journal (OJ) of the European Union and therefore provide a presumption of conformity with certain Essential Health and Safety Requirements (EHSRs) of the Machinery Directive 2006/42/EC.

There are many minor changes in this standard as you would expect. After some more analysis we will be releasing a full summary of the changes and their effects on guarding design and adaption in a full comparison document. You can pre-register for this on our newsletter to receive it first.

One of the changes relating to electrically conductive parts is explained below.

ISO/FDIS 14120:2015(E), 5.13 Guards with electrically conductive parts

Where guards are made of electrically conductive material and used in electrically powered machines, they might need to be considered as “extraneous conductive parts of the machine” according to IEC 60204-1:2005,clause 8.

There was no corresponding clause in the outgoing EN 953; so…… is this a completely new requirement to the machine builder?

In a word no. The manufacturer of the electrically powered machine should have be already designing and building their machine using EN 60204-1 (2006); and this is reflected in the clause in EN 14120.

However the clause given in the guarding standard states that the guard may need to be considered as “extraneous conductive parts of the machine“. It does not take into account that the guard may already be considered as an exposed conductive part of the machine rather than extraneous conductive parts. The determination as to how to classify the guard is dependent upon the limits of the electrical machine and where the guard is positioned in relation to it.

EN 60204-1 already requires that the risks associated with the hazards relevant to the electrical equipment shall be assessed as part of the overall requirements for risk assessment of the machine. So, the designer of the machine, the person modifying the machine for integration or in the case of adaption then the user, shall complete a risk assessment to determine what requirements are applicable. This risk assessment shall determine whether conductive parts (including guards) are either exposed conductive parts OR extraneous conductive parts.

exposed conductive part – conductive part of electrical equipment, which can be touched and which is not live under normal operating conditions, but which can become live under fault conditions AND

extraneous conductive part– conductive part not forming part of the electrical installation and liable to introduce a potential, generally the earth potential.

This would require the designer to firstly determine the limits of the “electrical machine”. An illustration of given in figure 1.

If we consider load 1; this motor is supplied by a cable with supplementary insulation. The motor would be considered as an exposed conductive part as a fault in the basic insulation of the supply cable at the motor or a fault in the motor windings would result in the motor becoming live. A protective conductor from the MET (PE terminal) to the motor would be selected taking account of the protective device used to ensure automatic disconnection of the supply within a sufficiently short time to limit the duration of a touch voltage to a time within which the touch voltage is not hazardous.

The frame of the machine (including guards A & B) would not be considered an exposed conductive part as the electrical machine ends at the motor. Instead it would be considered as an extraneous conductive part and as such the designer would have to consider whether equipotential bonding is required.

If we now consider load 2, this time the motor is supplied by a cable with only basic insulation routed through metal trunking and attached to the guards of sections C to F. This time the motor and the guards would be considered as exposed conductive parts as a fault in the basic insulation of the supply cable along its length up to the motor or a fault in the motor windings would result in the motor / guards becoming live. Protective conductors from the MET (PE terminal) to the motor and the guards would be selected taking account of the protective device used to ensure automatic disconnection of the supply within a sufficiently short time to limit the duration of a touch voltage to a time within which the touch voltage is not hazardous.

For more information on the design and documentation requirements of EN 60204-1 you could check our guide to the contents of the technical construction file for electrical machines