Boxes

By J. M. Woodgate B.Sc.(Eng.) C.Eng.  MIET SMIEEE FAES Hon FInstSCE MIOA

Boxes

Many people are familiar with 'IP ratings' – the coding that tells you what can and cannot get inside a box.  This is based on IEC standard IEC 60529, quite old as you can see by the number (we are above IEC 63000 now), but it was last updated in 2013, and the current edition also includes five corrigenda.

The Code consists of four or five characters – IP (the meaning of which is subject to differing opinions!), two digits and an optional letter. There used to be provision for a third digit, but that is no longer used. It was replaced by a different 'IK Code', embodied in IEC 62262, which is still current despite some Web-based 'information' to the contrary.

The first digit indicates protection against access to hazardous parts (electrical, mechanical, thermal…). The second digit indicates protection against the entry of water, by spray, splash, jet or immersion. The protection against other liquids might be different; for example, liquids with high surface tension can penetrate very thin slits, and whether the liquid wets or repels the material of the box is also a factor. There are six optional letters, but what they indicate is not easy to summarise; one has to look at the standard. There is a additional letter (K) that comes from ISO 20653, a version of the Code that applies to road vehicles and includes a test with strong hot-water jets. In the USA, there are 16 NEMA codes, which correspond roughly to some IP codes; there are 16 because the NEMA code doesn't separate the threats as the IP code does.

These Codes don't say anything about EMI getting into or out of the box. For that we have to look at the standards produced by IEC SC48D, plus one produced by IEC SC77C.

Mechanical structures for electrical and electronic equipment

That's the title of SC48D. It doesn't only deal with EMI but with the mechanical dimensions, such as the 19 inch rack and panel system. There are 37 current publications:

IEC 60297-3-1xx (multi-section): These are 11 standards about the 19 inch system;

IEC 60917-x-x (multi-part): These are eight standards about modular enclosures;

IEC 61587-x-x (multi-part): These are six standards about environmental and safety aspects, and here we find EMI standards;

IEC 61969-x (multi-part): These are about outdoor enclosures;

IEC 62194: How to measure the thermal performance. Keep it COOL!

IEC TS 62454: Design guide for water cooling. Keep it even cooler!

IEC 62610-x (multi-part): Some of the five Parts are TS, some full standards. They are all about thermal management.

IEC 62966-1: This is the first very new (at the time of writing) Part of a future multi-part and is about 'aisle containment', which is a way of improving the efficiency of cooling systems in large racked ITE installations by converting the aisles between the rack cabinets into enclosed 'hot air' and 'cold air' ducts.

The EMI standards

IEC 61587-3:2013 Edition 2.0 (2013-02-06)

Mechanical structures for electronic equipment - Tests for IEC 60917 and IEC 60297 - Part 3: Electromagnetic shielding performance tests for cabinets and sub-racks

This standard covers 30 MHz to 3 GHz, and gives details of the test set-up. The transmitting antenna is placed inside the enclosure, because it can be much smaller than the receiving antenna, which is either biconical or log-periodic. Measurements may be made on an OATS or in a semi- or fully-anechoic chamber.

IEC 61587-4:2012 Edition 1.0 (2012-08-13)

Mechanical structures for electronic equipment - Tests for IEC 60917 and IEC 60297 series - Part 4: Combination of performance levels for modular cabinets

This standard provides the combinations of different degrees of protection for cabinet

systems regarding IP code, climate levels, static and dynamics load tests, electromagnetic

shielding and seismic requirements. Some of the requirements are described as codes in IEC 61587-1. The use of Part 4 is intended to make a systematic approach to specifying all the requirements for an enclosure for a particular application as easy as possible. This is achieved by the use of simple tables.

IEC 61000-5-7:2001  Electromagnetic compatibility (EMC) - Part 5-7: Installation and mitigation guidelines - Degrees of protection provided by enclosures against electromagnetic disturbances (EM code)

This standard describes performance requirements, test methods and a classification for degrees of protection provided by empty enclosures against electromagnetic disturbances, for frequencies between 10 kHz and 40 GHz. The shielding effectiveness is measured prior to the installation of internal parts.

Shielding effectiveness is defined as field strength at a given point without the enclosure divided by field strength with the given point inside the enclosure. (It might not be the same at every point inside the enclosure, especially at 40 GHz.)

The EM code consists of 'EM' and four numbers or place-holder 'x', and is embodied in two tables:

As an example, EMxx456 indicates effectiveness increasing from =40 dB at 30 MHz to =60 dB from 10 GHz to 40 GHz