How to specify the ‘right’ Low Voltage Motor Control Centre

How to use the Low-voltage switchgear and controlgear “Guidance to specifying assemblies” (IEC/TR 61439-0) to specify a Low Voltage (LV) Motor Control Centre (MCC) with the correct features for his particular application(s). The IEC standard covering LV switchgear does not focus on the specification of the job. It covers the technical aspects of the assembly. For different applications, different solutions can be adopted and the standard has the flexibility to accommodate these differences. This flexibility, however, comes at a price since it means the specifier of the switchgear has to provide the manufacturer with substantial amounts of information if the manufacturer is going to offer the “right” switchgear for the application. Most specifiers are familiar with “headline” values such as short circuit withstand currents and times. What they may be less familiar with are the implications for both cost and size of the switchboard for what might appear to be minor requests. Examples of this are diversity factor, ingress protection, form of separation and ambient temperature. Therefore before starting to specify an MCC it is sensible for users to first be clear what features they require and why. The most common key functional requirements of continuous process industry users are very high reliability and the ability to work on spare or faulty motor starters safely while the remaining circuits remain energized. High levels of safety (by design) in terms not only of degree of protection from direct contact but also the ability to withstand an internal arc are often seen as mandatory. The following text describes the features that need to be specified and explains the implications when the specification exceeds the minimal values stated in the IEC standard.