Ticari Bina Elektrik Tesisatı için Önerilen Maksimum Direnç

Ticari binanızda elektrik muayenesi yapmak üzere bir kişi gelir. Mühendis, toprak elektrodunun 22 Ohm olarak test edildiğini bildirir. Yakındaki biri, NEC 25 Ohm’a izin verdiği için her şeyin yolunda olduğunu söyler. Ancak ekipman üreticisinin el kitabı, hassas elektroniklerle çalıştığı için maksimum 5 Ohm gerektiğini belirtir; bu nedenle devreye alma mühendisi sizin için yeni olan bir spesifikasyona atıfta bulunur. Hangi değere uyulmalıdır? Sorulan soru, ticari elektrik alanında çalışanlar için çok yaygındır, ancak cevap ölçtüğünüz standarda bağlıdır.

Bu yazı, doğru olan kesin sayılarla cevabı hemen gösterir – hayali sayılar kullanılmayacaktır. NEC’in tam olarak ne istediğini, IEEE’nin ne dediğini, “25 Ohm kuralının nasıl çalıştığını” ve neyin güvenli kabul edildiğini öğreneceksiniz.

Hızlı bir cevap arıyorsanız, tüm ticari binalarda topraklama için maksimum direnç konusunda evrensel olarak geçerli net bir sayı yoktur çünkü bu cevap hangi standarda ve sistemin hangi bölümüne atıfta bulunduğunuza bağlıdır. NEC (NFPA 70) bir bina için genel maksimum direnç belirlemez, ancak 25 ohm kombinasyonu (aslında 250.53“te belirtilmiştir) sadece topraklama çubuğu veya plakası için pratik anlam taşır ve sadece 1 çubuk veya plakanın topraklama elektrodu için yeterli olup olmadığını görmek için kullanılır (yani, 1 elektrodun 25 ohm veya altında olması durumunda ikinci elektrodun kullanılmaması uygundur). Büyük ticari ve endüstriyel sistemler için IEEE’nin Std 142 (”Green Book”), birincil topraklama elektrodunun toprağa direncinin 1-5 ohm aralığında olması gerektiğini belirtir ve sektör uygulaması (NFPA/IEEE önerileri, NETA ATS uygulamaları, telekom endüstri standartları vb.) ticari binalar için yaklaşık 5 ohm ve hatta trafo merkezleri ve üretim tesisleri için 1 ohm hedeflemektedir. Özetle, NEC’in minimum gereksinimleri, ticari binalar için 1-5 ohm aralığında IEEE tarafından talep edilen performansla ilgili değildir; bu aralık sektörde yaygın olarak kabul edilen hedef değerdir ve 25 ohm üzeri ticari topraklama elektrodu için zayıf ve kabul edilemez olarak görülür.

Doğru olalım, çünkü yanlış bilginin kaynağı burasıdır. İlgili sayılar ve kaynakları:

Standart / Rehberlik	Değer	Uygulandığı Alan
NEC 250.53 (NFPA 70)	25 ohm	Tek çubuk/plaka elektrod — direnç testi 25Ω veya daha az ise ikinci elektrod gerekmez
NEC 250.53 varsayılan	İki elektrod	Çubuk/boru/plaka elektrodlar varsayılan olarak iki adet; her ikisi takıldığında 25Ω testi gerekmez
IEEE Std 142 (Yeşil Kitap)	1–5 ohm	Büyük ticari/endüstriyel sistemler için ana topraklama elektrodunun önerilen toprak direnci
IEEE Std 80	5–1 ohm	Trafo merkezi / üretim tesisi topraklama şebekeleri
NFPA/IEEE sektör rehberi	5 ohm veya daha az	Yaygın olarak belirtilen ticari hedef; telekom endüstri standardı
NETA ATS kabul testi	5 ohm / 1 ohm	≤5Ω büyük ticari/endüstriyel; ≤1Ω üretim/iletim istasyonları
Ekipman üreticileri	<1 ila 5 ohm	Hassas elektronik / tıbbi tanı ekipmanı spesifikasyonları

Önemli gerçek: NEC’de ticari bir topraklama için “25 ohm veya daha az” olma zorunluluğu yoktur. 25 sayısı yalnızca tek bir elektrottan yeterli mesafe olup olmadığını belirlemek için dahil edilmiştir. IEEE 142 ve NETA önerileri ve uygulamaları gibi performans gereksinimleri 1 ile 5 ohm arasında bir değer önermektedir – en yaygın kullanılan sınır ise 5 ohmdur.

NEC 25-Ohm Kuralı — Gerçek Anlamı

NEC Bölüm 250.53(A)(2) (ve ayrıca 2005 NEC Bölüm 250-56) uyarınca, kod iki topraklama elektrodu kurulmasını gerektirir. Ancak, tek bir elektrodun 25 ohm veya daha az toprak direncini sağladığı durumlarda izin verildiğine dair önemli bir istisna vardır. Ne yazık ki, bu genellikle sadece bir elektrodun kurulup direncinin test edilmesi ve eğer ilk elektrod 25 ohm direncini karşılamazsa ikinci elektrodun eklenmesi gerektiği şeklinde yanlış yorumlanmaktadır.

Aslında, dağılım şu şekildedir:

Varsayılan = en az 6 feet arayla iki elektrodun kurulması.
İzin verilen = toprak direncinin 25Ω veya daha az olduğunu doğrulayan uygun bir direnç testine tabi tutulan tek elektrod.

İkinci elektrodun kurulması durumunda, kombinasyonun 25 ohm koşulunu karşılaması gerekmez — çünkü ikinci elektrod kurulumuyla NEC gereksinimlerini yerine getirmiş sayılır.

IEEE 142 (Yeşil Kitap): 1–5 Ohm Önerisi

“Ticari bir yapı için önerilen direnç nedir?” sorusu, IEEE Std 142 olarak bilinen Yeşil Kitap’tan otoriter bir mühendislik çözümü alır. Önerilen değer, büyük endüstriyel ve ticari sistemlerde ana topraklama elektrodu için 1 ila 5 ohm arası toprak direncidir.

Bu direnç değerinin 25 ohma tercih edilmesinin nedeni, ticari tesislerin genellikle hassas elektroniklerle donatılmış olması ve bu nedenle sadece kod minimumuyla sağlanamayan çok stabil ve düşük empedanslı bir toprak referansına ihtiyaç duymasıdır. Yukarıda belirtilen düşük toprak direnci şunları sağlar:

Arızadan kaynaklanan yüksek akımın kaynağa daha hızlı dönmesi
Arıza sırasında ekipman kasasında daha az voltaj artışı
Daha iyi yıldırım dalga sönümlenmesi
Daha az elektriksel gürültü

Özetle, kod 25 ohm gerektirir; mühendislik terimleriyle iyi performans için gereken değer 1-5 ohmdur.

Sektör Uygulaması: Ticari Hedef Olarak 5 Ohm

Sahada, çoğu kişinin ticari sistemler için duyacağı bilgi “5 ohm veya daha az”dır. Bu, aşağıdaki çeşitli ancak tamamlayıcı kaynaklardan gelir:

NFPA ve IEEE’nin 5.0 ohm veya daha azı iyi bir hedef olarak önerileri
Telekomünikasyondaki yaklaşımlar (topraklama ve bağlama için 5 ohm veya daha az)
NETA’nın kabul testleri standartları (ticari/endüstriyel sistemler için ≤5 ohm; enerji santralleri/iletim istasyonları için ≤1 ohm)
farklı ekipman üreticileri tarafından verilen rakamlar (hassas elektronikler için 1 ila 5 ohm arasında)

Farklı mühendisler ve test uzmanlarının ortak tavsiyesine göre, ticari binalar için 5 ohm veya daha az iyidir, alt istasyonlar için ise ortalama değer 1 ohm ve altıdır. Bu NEC tarafından bir gereklilik değildir, ancak birçok tasarımcının takip ettiği ve testlere tabi olan en iyi uygulamalardır.

Güvenli Bir Aralık Var Mı? Sayıları Anlamak

Here’s how to interpret ground resistance readings in practice:

Resistance	Interpretation
< 1 ohm	Excellent — typical of large grids, substations, high-density electrodes
1–5 ohm	Good — the recommended range for commercial/industrial (IEEE 142)
5–10 ohms	Acceptable for many installations, but worth improving for sensitive loads
10–25 ohms	Meets the NEC single-electrode threshold but marginal for commercial electronics; investigate
25–100 ohms	Poor — above the NEC single-electrode limit; likely needs multiple electrodes or treatment
> 100 ohms	Very poor — unacceptable for any permanent installation; rework required

The key warning is as follows: there is not one universal “safe” value that applies to each situation from a safety perspective — it is also influenced by different factors such as earth fault current, soil conditions and the actual circuit. The quality of electrode grounding resistance is only a part of the problem; the integrity of the whole grounding system is just as important. A perfect electrode of 2 Ohm is useless if the ground conductors of the devices are crooked or corroded.

Is 20 Ohms Bad? Interpreting Your Measurement

20-ohm resistance levels are not viewed positively in the commercial building construction code, but there are also no code violations associated with that value, hence:

NEC standard: This level is less than the maximum single-electrode value determined by the above legislation. Nevertheless, 20-ohm resistance level is still close to exceeding the standards, depending on the seasonal increase and decline of moisture level in the soil.
IEEE 142 standard: Based on this standard, the resistance level of 20 ohm exceeds the recommended value of 1-5 ohm, meaning that some of the electronic equipment, such as those preventing surge operations, might be damaged because of the improper grounding level.

In practical terms, any commercial organization will make sure the grounding meets the set standards.

Conclusion: Generally speaking, 20 ohm of resistance may not threaten the life of the building, but it is still too high for a commercial building; therefore, it needs to be improved.

Other Commercial Requirements: Receptacles & Bonding

The ground resistance is a part of commercial electrical safety. The National Electrical Code (NEC) requirements are as follows:

The characteristics of commercial receptacles (NEC 406, 210.52, 210.8): The commercial receptacles shall be specified to be 125V,15A and/or 20A types, GFCI protection is necessary to be provided at certain locations (bathrooms, rooftops, kitchens, outdoor locations etc. per 210.8(B)), all the receptacles will be connected to the metal box and each receptacle shall be ensured to be connected to the grounding conductor.
Bonding process (NEC 250.102-250.104): It is essential to understand that all the metallic parts which can become energized like piping, framed equipment, structural steel, pumping equipment must be bonded to the grounding system. It’s worth noting that bonding continuity testing will be carried out separately from electrode resistance (point-to-point resistance should not exceed 0.5 ohm when equipment is connected to the grounding system).
Grounding electrode conductor (NEC 250.66): The grounding electrodes must be sized according to the service doesn’t exceed the maximum diameter and should connect the grounding system to the service equipment.

If you’re also verifying the protective devices on the circuit, our explanation of circuit breakers vs ground-fault circuit interrupters clarifies which protection does what in a commercial panel.

What Counts as High Electrical Resistance?

Context plays a role in what “high resistance” means:

When measuring grounding electrodes, it means greater than about 25 ohms (for instance, 100 ohms or greater is a very high value). The “ideal” resistance level is 1-5 ohms.
In terms of continuity of grounding, anything more than 0.5 ohm indicates a fault. The connection between equipment and ground should be almost zero.
With respect to insulation resistance, a high resistance value means a good resistance since it is measured in mega and gigohms where anything below 1 MΩ indicates a high probability for a breakdown.

In conclusion, “high resistance” almost always means a problem (for grounding) but it can mean a good situation (for insulation). Always make sure what exactly one is talking about when using a term “high resistance”.

Factors That Affect Ground Resistance

Ground resistance is not a constant value. It varies with circumstances:

Soil resistivity: Clay/loam (10–100 Ω·m) has lower resistivity than sand/gravel (1,000–10,000 Ω·m).
Moisture content: Wet soils have lower resistivity while dry weather increases resistance enormously.
Temperature: Cold soil has much higher resistivity.
Electrode depth and cross section: Longer electrodes and bigger plates go deeper into soil.
Number of and separation between electrodes: The more electrodes you have and the farther apart they are, the lower the resistance will be.
Electrode material and corrosion: The longer the electrodes stay in the ground, the more corrosion there is, and resistance will increase.

How to Achieve a Low-Resistance Ground

In case your commercial grounding is excessively high, here’s a ranked list of the most cost-effective solutions:

Add rods: Adding a second ground rod, placed 8 feet away from the first, will greatly reduce the resistance. Continue adding additional rods until the correct resistance is achieved (the benefit generated decreases with each added rod).
Use larger or longer rods: Using a longer rod (10-foot) or thick rods or plate electrodes will help reach deeper and less resistant soils.
Improve soil connection: Backfilling conductive materials or chemical additives around the rod will improve its connection with the soil.
Create ground rings or grid: Underground copper conductor encircling the structure creates a bigger area of connection (this solution is widely used in commercial/industrial field).
Use concrete-encased electrodes (Ufer grounds): Concrete-embedded rebar and conductor show excellent and reliable conduction properties.
Connect to the building’s steel parts and water pipe: Connecting to the metal structural steel and underground water pipes (if any) gives additional ways of conduction.

Be sure to test every time you change the system, but keep in mind that the NEC does not require retesting the two-electrode system.

How Ground Resistance Is Tested

Ground testing is to be carried out by persons qualified for this job. The main and widespread methods are:

Fall-of-Potential (more commonly known as three-point testing): The fall-of-potential method consists in the use of two temporary stakes and resistance measuring with increasing distance. It is the most precise technique used for small to medium installations.
Clamp-on (or inductive) testing: The clamp-on method involves the clamp around the rod/conductor so that the loop resistance can be measured without disconnecting the loop. It is advantageous for the already existing installations but requires the whole loop connected.
Two-point testing: The two-point method is used to make fast measurements against a known reference electrode: this technique is less accurate, thus used in rough calculations.

Testing is dangerous, since it usually requires isolation of the electrode, and fault current may be present, this job should not be done by an ordinary homeowner. For commercial buildings the commissioning and regular tests usually are performed by contractors or specialized companies in accordance with NETA standards.

Sıkça Sorulan Sorular

Which guideline outlines the acceptable maximum resistance for earthing in commercial installations?

NEC does not specify a single standard maximum value. The 25-ohm requirement outlined in NEC 250.53 applies only to the case of a single-electrode system, and not to the whole building. A major engineering standard is IEEE Std 142 (the Green Book), which suggests that the value of the main grounding electrode in large commercial and industrial installations should be in the range of 1-5 ohms. Normally, in practice, and accepting standards set by NETA, the value of 5 ohms is satisfactory for a commercial building (1 ohm for a substation). In general, NEC shows the limit according to the requirements of the code, IEEE 142 provides the acceptable range according to the engineering requirements, and then 5 ohms emerges as the accepted practice.

Is 20 ohms of resistance in a ground bad?

A resistance of 20 ohms is not hazardous in itself but is borderline acceptable for commercial buildings. While the NEC’s single-rod criterion of 25 ohms is passed, the number is above the level recommended by IEEE 142 for commercial installations, which is 1－5 ohms, as well as the commonly accepted level of 5 ohms. With that level of grounding, the fault current is returning in a slower manner and not providing remarkable surge protection or reference grounding for sensitive electronics as well. And since it is just a step away from going above the code threshold, the level may go up as a result of soil condition changes during the seasons. Hence, the grounding system should be improved in the case of commercial installations.

What are the NEC commercial receptacle requirements?

All commercial receptacles have a voltage rating of 125 volts to 15 or 20 amps as stated in NEC 406, and their minimum amount and spacing rules can be found in 210.52 for general areas. GFCI protection is mandatory in given commercial spaces under NEC 210.8(b), such as bathrooms, kitchens, rooftops, outdoors, and garages/service areas. All receptacles must be provided with an equipment grounding, and metallic boxes are to be grounded. In many commercial child-occupied places, tamper-resistant receptacles are a must, while some types of occupancies have their particular rules for provided receptacles.

What is a high electrical resistance?

“High electrical resistance” depends upon what measurement is at hand. With regard to grounding electrodes, anything over approximately 25 ohms counts as high, while numbers over 100 ohms are considered very high. Ideally, the resistance should be somewhere between 1 and 5 ohms. Concerning equipment grounding continuity, the electrical resistance should not exceed 0.5 ohm when measuring the equipment and the ground. In insulation, a high resistance is a positive sign. For insulation to be considered healthy, it must have a resistance of millions of ohms (MΩ) or even billions of ohms. A resistance level of less than 1 MΩ indicates a breakdown with leakage danger.

Referanslar

Conclusion

The NEC’s two figures — 25 ohms and 1-5 ohms as per the IEEE 142 guide cover the maximum allowed electrical ground resistance of a commercial building. While the NEC does not consider for a commercial building the value of under 25 ohms that would indicate compliance with the code as a performance standard, it depends on whether a single electrode is sufficient. From this standpoint, the ideal value for reliable and safe performance of commercial installation would be in the range of 1-5 ohms, as noted by IEEE, thus facilitating fast failure elimination, effective surge protection, low noise level for commercial electronics.

Use proper fall-of-potential testing to get a good measurement and tally it against the range table (as good value is under 5, value ranging from 5 to 25 is marginal, value over 25 is below the acceptable level); you may need to add more or longer electrodes to the grounding system unless you find the readings satisfactory. Finally, do not forget that the resistance of electrodes is only half of the story and the continuity of bonding and grounding must also be accounted for.