Smart solutions overview of street light system in TI grounding system

There are many of electrical equipment which can be accessed by the public; however, some of them cannot be physically controlled by the operator. One of them is street light system which needs to be protected in order to prevent shock hazards under the fault conditions. Generally, street lighting systems are distributed in a large area including urban and sub urban. If fault happens, the metal case of these systems might raise a potential hazard which could lead to expose people to shock hazards. The typical protections are bonding and grounding to reduce the risk to an acceptable level. This technical literature review will be based on an IEEE paper titled “Electrical Safety of Street Light Systems” by G. Parise, L. Martirano, and M. Mitolo (2011) [1]. This will be demonstrated firstly by explaining the terms and content involved, secondly by discussing the work in the paper and finally presenting the contribution of the paper and the key learning point. In this article, the authors organize the content in eight sections. In the first five sections, the authors present the street lightning system in general, the protections used, and variation of street lighting system according to the grounding systems applied. In section six, the authors describe an example in US. The main content is in section seven where the authors explain about proposed solutions which mainly regarding to improve TI system. Finally, the authors put the conclusion and suggesting the future area of research.

Street lighting systems are a common case of distributed low-voltage load in wide areas. It usually protected by the same devices. Electrical utilities are commonly owned and operate this system. IEC 60364 [2] has set the standard of Class II to be used in street lighting systems. The purpose is to protect the systems against indirect contact. The term of Class II in here include the wiring systems, the light fixtures, etc. which have double reinforced insulation. Under normal condition and proper maintenance system, the risk of insulation break down is significantly low. However, the authors here mention the major issue that it might fail during its life cycle. It could be mainly caused by improper maintenance due to the wide area covered and also can be a result from extreme conditions such as vehicle impacts or animal intrusion into poles. Therefore the authors propose possible alternatives for the protections systems beside that are already mentioned in IEC standards. The authors suggest increasing the safety level of Class II metal poles by applying a special circuitry and bonding connections. The purpose is for providing continuous information about the double insulation status.

Section one until five are actually already covered by the authors in their previous paper “Grounding of Distributed Low-Voltage Loads: the Street Lighting Systems” published by IEEE in 2010 [3]. It includes protection against indirect contact and types of grounding in section two. This part is mainly explain protections in low voltage system related with grounding systems which can be carried out from a common earthing electrode to source and load; known as TN system; or through independent grounding systems; known as TT system. The authors’ explanations will mainly on improving TI system which is not formally defined in IEC standard. In TI systems, there is direct connection of source neutral to earth and exposed conductive parts (ECP) are not connected to the ground. All grounding systems are based on reference [4]. This includes also the TN-S; with separated neutral wire and protective conductors (PE); and TN-C-S systems; with combination of separated and a combined neutral wire and PE as a single conductor known as PEN. The grounding systems mentioned are based on outdoor installations as mentioned in reference [5]

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In street lighting in TT system environment, the authors conclude that residual current devices (RCD) must be employed. This was because of the protective overcurrent device which might not trip within the maximum allowable times due to long time pickup. TT systems are generally used in Europe. In North America, which commonly used TN-C-S systems, the major concern is that this system will be likely seen as TN-S in the downstream user which should not allowed the condition of PE passing through the RCD’s toroid, otherwise the fault current will not be sensed and the protection will likely fail. In TI street lighting system, at the source, the neutral point is grounded whereas all of the electrical components should be Class II and ungrounded. This system is generally used in Italy where the electrical distribution line will consist of four-wire three-phase or two-wire single-phase. In this system, the protection is carried out by overcurrent protective device. The authors emphasised that is the double insulation fails, the systems would be likely seen as TT systems which the overcurrent protective device might not be able to clear the faults due to the absence of RCD. The metal pole would become permanently energised until the operator maintenance officer detects the fault. There will be a risk of exposing people to shock hazards. This was considered as the drawback of this system thus the maintenance process would carry a higher responsibility for the operator. Addressing to the issue of street lighting in TI systems, the authors suggest two types of solutions which can be applied individually or as a combination to improve the system. Those can be approached by putting an additional level of protection which will be applied to all the component of the street lighting system and applying a smart solution localised within the panel board.

There will be three arrangements of smart solutions to be discussed. The first is by putting an additional level of protection. Double insulation provides basic protection but it might be difficult for detecting its fault. Additional level of protection is in the form of adding the residual current relay within the protective device. This will be combined with one or two of the followings:

- Adding multi conductor cable with integrated protective conductor to reduce accidental connection risk.

- Adding buried base grounding wire connected to each metal pole.

The arrangement can be seen in figure 1. In this arrangement, double insulation will be as the first level of protection. If it fails, the supply will be disconnected and/or an alarm will be activated. In the case of TI system, this arrangement will likely have two levels of protection which act as main and backup protection whenever the main fails. The second arrangement is the application of “smartpanel” with an insulation monitoring device. This can be seen in figure 2. In TI system, it will be practically difficult to set up the insulation test circuit. To address this, the authors propose the application of a “smartpanel” with a special insulation monitoring device (IMD) inside it. The IMD will consist of a permanent testing circuitry and a transmitter system. The testing circuitry consists of a double insulation test switch and a ground test electrode which permanently installed in and accessible inspection channel. The transmitter system will send the testing data to the maintenance centre which leads to an effective maintenance activity. The transmitter system can be a GSM, GPRS or power line communication (PLC). Although this arrangement will add extra cost to the owner, it will significantly reduce the maintenance activities and simultaneously keep the public safe thus it will save the total cost in the future while still keep maintaining the safety matter.

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Figure 1: The grounding electrode only operates upon failure of the double insulation (TT system).

Figure 2: Panelboard equipped with the test switch and accessible ground test electrode

The last arrangement is by applying the same “smartpanel” but now equipped with residual current monitoring device (RCMD). The residual current relay will assure of local alarm warning when fault happens. Notification will be immediately sent to the maintenance centre. This allows the condition

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without disconnecting the lighting system supply. In comparison of those three arrangements, the last arrangement mentioned above has the lowest cost related to maintenance while it is most expensive cost in standard TI system. The complete comparison is shown in table 1.

Table 1: Comparison among the different smart solutions

The TI system is proven to be an efficient approach in keeping people safe from electric shock without disconnecting the supply. This system is mainly suitable in high pedestrian and vehicular circulation. Even though the failure possibility of the double insulation is very low, the authors still see that case as a form of risk since it is not zero. Therefore, the risk should be lowered by applying additional protection system. The adoption of grounding electrodes for Class II light pole systems will bring benefit when there undetected failure occurs.

The key learning points of this paper is that the grounding system as a form of a protection system should aim to reduce the risk of exposing people to electric shock hazards thus its performance must be evaluated to ensure the safety level to reach to the highest point. TT, TI and TN systems have each own different characteristic of fault loop. TT and TI consist of the actual earth which produces difficulties for indirect contact protection by disconnection of the supply based on overcurrent devices. In real condition with real example as it happens in Italy with TI system, the additional protection system as discussed above is proven to reduce the hazard risk therefore it should be implemented. The main problem of street lighting in TI system can be addressed with several alternatives of “smart solutions”. The general question asked by the operator is usually based on the cost of implementation and the benefits they will get from it. It can be seen that the implementation will need initial investment on additional protection system as mentioned above such as test switch, test electrode and the transmitter interfaces. The authors report that the additional expenses would not exceed half of the panelboard cost and claim that the operator will experience significant cost reduction due to maintenance activities. The options in table 1 can be chosen by the operators.

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In conclusion, this paper present a group of grounding system variations applied in street light system which informs the reader about potential hazards mainly in TI system. For the street light electric distribution system, TI systems are promoted by IEC, which does not require or utilize the RCDs thus prevent nuisance tripping. The proposed solutions increase the safety level offered by Class II equipment as it is mentioned in IEC standard and can be used as guidance for utilities to improve their system. This article is mainly focused on metal pole, therefore, the future research should proposed more general solutions in the in the case of concrete or wooden poles as well as in the case of light systems in high-resistivity soils.

Reference List

[1] G. Parise, L. Martirano, M. Mitolo, “Electrical safety of street light systems”, IEEE transactions on power delivery, vol. 26. No. 3. Pp. 1952-1959, July 2011.

[2] Low-Voltage Electrical Installations—Part 4-41: Protection for Safety–Protection Against Electric Shock, 2005, Ed.5, IEC 60364-4-41, 2005.

[3] G. Parise, L. Martirano, M. Mitolo,T. Baldwin, S. Panetta “Grounding of Distributed Low-Voltage Loads: the Street Lighting Systems”, IEEE, 2010

[4] M. Mitolo, Electrical Safety of Low-Voltage Systems. New York: Mc-Graw-Hill, 2009.

[5] M. Mitolo, “On outdoor lighting installations grounding systems,” in Proc. IEEE Ind. Appl. Soc. 41st Annu. Meeting, Conf. Rec., Tampa, Fl, Oct. 2006, vol. 5, pp. 2224–2229.

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