Chapter 12 - System Integrity and Safety

Chapter 12 - System Integrity and Safety

12-1. Objectives

Upon completion of this chapter, you will know ” how to

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12-2. Introduction

In addition to being able to design and program an efficient working control system, it is important for the system designer to be well aware of other non-electrical and nonsoftware related issues. These are issues that can cause the best and most clever designs to fail prematurely, work intermittently, or worse, to be a safety hazard. In this chapter, we will investigate some of the tools and procedures available to the designer so that the system will work well, work safely, and work with minimal down-time.

12-3. System Integrity

It is obvious that we would never consider exposing a twisted copper wire connection to the outdoor weather. Surely, the weather would eventually tarnish and corrode the connection, and the connection would either become intermittent or fail altogether. But what if the connection were outdoors, but under a roof - say a carport? Would a bare twisted wire connection be acceptable? And what if the same type of connection were used in a ceiling light fixture for an indoor swimming pool? Surely the chlorine used to purify the pool water will have some adverse affect on the twisted copper wires.

In general, how does a system designer know how to ward off environmental effects so that they will not cause premature failure of the system? One solution is to put all of the electrical equipment inside an enclosure, or electrical box. But how do we know how well the electrical box will ward off the same environmental effects. Can we be sure it will not leak in a driving rainstorm? The answer lies in guidelines set forth by the National Electrical

Manufacturers Association (NEMA), and the International Electrotechnical Commission

(IEC) regarding electrical enclosures. NEMA is a United States based association, while

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Chapter 12 - System Integrity and Safety

IEC is European Based. Both have set forth similar standards by which manufacturers rate their products based on how impervious they are to environmental conditions. NEMA assigns a NEMA number to each classification, while IEC assigns an IP (Index of Protection) number. It is possible, to some extent, to be able to cross reference NEMA and

IEC classes; however, there is not an exact one-to-one relationship between the two.

NEMA and IEC ratings are based mostly on the enclosure’s ability to protect the equipment inside from accidental body contact, dust, splashing water, direct hosedown, rain, sleet, ice, oil, coolant, and corrosive agents. Since the designer knows the environment in which the equipment is to be used, it is relatively simple to lookup the required protection in a NEMA or IEC table and then specify the appropriate NEMA or IP number when purchasing the equipment. Generally speaking, the NEMA and IP numbers are assigned so that the lower numbers provide the least protection while the highest numbers provide the best protection. Because of this, the cost of a NEMA or IEC rated enclosure is usually directly proportional to the NEMA or IP number.

Consider the NEMA enclosure rating table below. If for example, we needed an enclosure to protect equipment from usual outdoor weather conditions, then a NEMA 3 or

NEMA 4 enclosure would be acceptable. However, if the enclosure were near the ocean or a swimming pool where it would be exposed to corrosive salt water or chlorinated water splash, then a NEMA 4X would be a better choice. In a similar manner, we can conclude that underwater equipment must be NEMA 6P rated, and that an enclosure that is to be mounted on a hydraulic pump should be NEMA 12 or NEMA 13.

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Chapter 12 - System Integrity and Safety

NEMA Enclosure Ratings

It would seem logical to simply use NEMA 6P for everything (except oil and coolant exposure). However, the very high cost of NEMA 6P enclosures prohibits their use in nonsubmerged applications. Therefore, because of cost constraints, it is also important to avoid overspecifying a NEMA enclosure.

IEC enclosure numbers address environmental issues as does NEMA. However, the IEC numbers also address safety issues. In particular, they specify the amount of personal protection the enclosure offers in keeping out intrusion by foreign bodies, such as hands, fingers, tools, and screws. IP numbers are always two-digit numbers. The leftmost

(tens) digit specifies the protection against intrusion by foreign bodies while the rightmost (units) digit specifies the environmental protection provided by the enclosure. The IEC IP number ratings are shown in the table below.

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Chapter 12 - System Integrity and Safety

IEC IP Enclosure Ratings

There is also a rating of IPx8 which is waterproof. There is no tens digit on this rating because since it is waterproof, is it also naturally impervious to any and all foreign objects. Also, since there is only one column 7 rating (which is IP67), it is referred to as either IP67 or IPx7. In the IP_2 column, “inclined water” refers to rain or drip up to 15 degree from vertical, and in the IP_3 column, spray water can be up to 60 degrees from vertical.

As some examples of how to use the IEC table, assume we wish to have an enclosure that will keep out rain (inclined water) and not allow tools to be pushed into any openings. Locating those items in the columns and rows, we find that an IP32 enclosure is needed. Additionally, an enclosure that will keep out hands and offers no environmental protection is an IP10 enclosure. Most consumer electronics products (stereos, televisions, VCRs, etc.) are IP40.

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