Grounding Practices for Traffic Control Systems

Transcription

1 2014 Grounding Practices for Traffic Control Systems Induced surge currents and transient over-voltage occurring on power and signal lines account for a large percentage of damage and mis-operation of traffic control systems if multi-grounded and unprotected. This Tuesday Traffic tip is from the May/June 2010 edition of the IMSA Journal, written by Alan Rebeck and Daniel Lawlor September 16, 2014

2 Grounding Practices for Traffic Control Systems By Alan Rebeck and Daniel Lawlor Introduction Induced surge currents and transient over-voltages occurring on power and signal lines account for a large percentage of damage and mis-operation of traffic control systems if multi-grounded and unprotected. These occurrences, lasting only a short period of time, can be produced by lightning, power companies switching feeders or capacitor banks, or load switching at customer facilities. Single-Point Grounding Problems like these can be avoided by implementing a single-point grounding system, following the National Electrical Code (NEC) when installing the safety (equipment ground) and grounding electrode systems, and the use of a properly designed and selected surge suppressor. (See Figure 1) often located near or mounted on a power pole. The NEC requires that all earth ground references be directly bonded to the original Neutral-Ground bond. Ground Ring. This system is comprised of a minimum #2 AWG, bare wire buried no less than 30-in. under the soil surrounding a structure or pad mounted equipment. The ring ground gives more of an equal-potential ground around the facility. It s almost always supplemented with earth ground rods. (See Figure 2) Figure 2 Figure 1 Concrete Encased Electrode. Metal bars encased in concrete in contact with the surface of the earth. Also known as an Ufer ground. The foundation installed for a traffic control cabinet constitutes additional earth grounding. (See Figure 3) Within a traffic control cabinet, this is normally accomplished for the enclosure and internal system components by use of an equipment ground bar connected to a local earth ground system. Lightning problems arise, however, when the required neutral-ground bond at the first disconnecting means is some distance away and referencing a different earth ground system, typically a driven rod at a power pole installed by the utility. An additional Neutral-Ground bond at the traffic control cabinet will cause many amps of current to be diverted to the equipment grounding conductor from the bond in the disconnect switch. Changes in this objectionable current create voltage spikes or transient overvoltages in the cabinets any time you have a fast changing current in a wire you have spiking. Earth Grounding Techniques The NEC allows a single-point grounding system to be connected to the earth in seven different ways: Rod and pipe electrodes. About 90 percent of all grounding electrode system installations are rod and pipe electrodes. Generally, this is comprised of an 8 to 10 foot stake driven into the earth that extends up and connects to the Neutral conductor at the first disconnecting means which in the case of a traffic control system will be a main disconnect switch Page 28 Figure 3 Other legal grounding electrode systems include: Grounded metal building frame Plate electrodes metal plates buried in the earth for a larger surface area Supplemented metal underground water pipe although the code doesn t allow the use of an underground water pipe by itself anymore, this can be used as a secondary or third connection to the earth Underground local metal structures (exception: gas piping). Continued on page 30

3 Case Study Findings: An electrical power, grounding and transient overvoltage protection inspection was performed on October 7, 2008 by RO Associates. for the City of Hamilton at the First Road and Mud Street intersection (Photo 1). The main objective of the inspection was to determine the cause of frequent lightning related damage, disturbance and degradation of electronic components of traffic control systems located throughout the city. Additional objectives were the recommendation of more effective grounding and bonding topologies and improved lightning surge protection. Continued from page 28 pliance with accepted installation practices, measured voltages, currents, harmonic distortion and electrical noise and inspected lightning and surge protection schemes. Test equipment used during the inspection included a power line analyzer/scope, AC/DC clamp-on ammeter and earth ground impedance tester. Assisting in the inspection was Daniel Lawlor, Project Manager of Traffic Electrical Systems for the City of Hamilton. The system inspected consisted of a TS-1 Fortran traffic control cabinet (Photo 2) and numerous traffic lights supported by metallic poles. Damage has primarily been sustained by the 120 Vrms power supply and data communication interfaces associated with the timer and conflict monitor. Page 30 Photo 1 Hamilton is situated on Lake Ontario and is rated as the third highest city in the country for lightning activity according to Environment Canada s website. In particularly active years the City has lost thousands of dollars worth of sensitive electronic traffic control equipment. For this reason the City has conducted an earth resistivity study, developed new grounding methods using conductive concrete as an enhancement material and has established new engineering standards for grounding and bonding traffic signal equipment. The City called upon RO Associates to conduct a case study and to provide recommendations which have factored into the development of its new standards. It is important to note that while there are many similarities, Hamilton is governed by the Canadian Electrical Safety Code rather than the NEC; furthermore, an Ontario Provincial Standard exists which specifies grounding and bonding recommendations for traffic signal equipment; and finally the Electrical Safety Authority in Ontario permits double bonding of ground and neutral for traffic signal controllers in order to allow manufacturers to comply with the NEMA TS-1 specifications for traffic controllers (the document shows a N-G bond in the power supply portion of the traffic controller circuitry). The reader is cautioned to bear in mind that local standards and codes may vary and take precedence. During the inspection the RO Associates Engineer checked for proper wiring of the AC power distribution, surveyed the grounding scheme for code compliance, verified com- A 30 Amp breaker in the traffic control cabinet is supplied 120 Vrms, single-phase power from a 30 Amp, fused disconnect switch located in an enclosure near a utility power pole located approximately 100 from the cabinet (Photo 3). There is no incoming data communications cable from a cabinet at another intersection to this particular cabinet. Photo 2 A neutral-ground bond to both a driven rod and a buried plate is installed at the 30 Amp disconnect switch, which is proper for a first disconnecting means. The phase and neutral from the disconnect switch to the cabinet are both #10 AWG, insulated, Photo 3 stranded conductors. The equipment grounding conductor is a #6 AWG, green insulated stranded conductor. Measuring at the cabinet (Photo 4), phase-neutral voltage measured low but within allowed limits at Vrms with total voltage harmonic distortion of 3.7% thd. Most manufacturers specify 120 Vrms + 5%/ -10%. IEEE Standard 519 allows a maximum of 5.0% thd for voltage harmonic distortion. Neutral-ground voltage measured 2.7 mvrms. Phase current measured 2.9 Amps with total current harmonic distortion of 7.2% thd. Current in the neutral conductor measured only Continued on page 32

4 Continued from page Amp due to an additional N-G bond at the cabinet also causing a ground loop current of 2.0 Amps to flow in the equipment grounding conductor from the disconnect switch. There is also a small clearance from the neutral bar to the cabinet frame by means of isolating washers. 1.2 and 0.9 Ohm. Impedance to earth from the nearby signal light pole measured 3.7 Ohms proving that it is not bonded to the traffic control cabinet earth grounding system. The U.S. National Electrical Code recommends a maximum of 25 Ohms for touch safety and telecommunications and PLC industry standards require a maximum of 5.0 Ohms for logic reference purposes. A filter type, surge protective device was installed for transient overvoltage and lightning surge protection of the AC power supplied to the cabinet. An MOV type, surge protective device was installed for lightning surge protection of a data communications interface if utilized. Summary: Storm related damage to the City of Hamilton traffic control system inspected can be attributed to: Lack of a local earth ground reference for the traffic control cabinet Photo 4 No current was detected in the grounding electrode conductor to the driven rod and plate at the 30 Amp disconnect switch, however, as previously cited 2.0 Amps was measured in the equipment grounding conductor to the cabinet due to a N-G bond in the cabinet. #6 AWG, insulated, stranded bonding conductors from the signal light poles were supposed to connect to the grounding electrode conductor at the disconnect switch but were not bonded to the disconnect switch or the cabinet. An equipment grounding conductor to an aluminum signal light pole nearby the cabinet was observed to be cut. There was no local earth ground reference for the traffic control cabinet other then the concrete pad. The equipment grounding conductor from the disconnect switch was properly connected to the equipment ground bar in the cabinet but this bar is not intentionally bonded to the cabinet enclosure. A DC resistance of 0.6 Ohm was measured from the safety ground bar to the enclosure. Standard NEMA TS-1 defined connectors and cable harnesses provide power and data communications to the timer and conflict monitor. The enclosures of the timer and conflict monitor are intentionally grounded only by means of small gauge, green insulated conductors grounding the connector shells. DC resistance from each enclosure to the ground bar measured 0.01 Ohm and to the cabinet enclosure measured 0.1 Ohm. The data communications surge protective device is grounded to the enclosure and the shield of the protected cable is connected to ungrounded pin 1226 on a terminal block. Previous soil resistivity data for this intersection, with various probe spacing from 0.5 to 15 meters, resulted in readings from 19 to 45 Ohm-meters indicating the existence of a fairly low impedance soil at this location. Using the three-point, 62% fall-of-potential method, impedance to earth from the disconnect switch and the cabinet respectively measured Page 32 Multiple neutral-ground bonds at the disconnect switch and the traffic control cabinet Lack of an adequate earth grounding topology for the intersection Absence of a conductor bond for local earth grounding of the cabinet enclosure Inadequate sized equipment grounding conductors for the enclosures of the timer and conflict monitor. Failure to bond the equipment grounding conductors for the signal light poles to the traffic control cabinet earth grounding system No policy for grounding of master and slave, traffic control cabinet, data communications cable shields Possible filter type only surge protective device (SPD) for transient overvoltage and lightning surge protection of the incoming AC power distribution The significant distance between the 30 Amp disconnect switches and the traffic control cabinets at both sites inspected does not represent a single point ground and is of concern. When lightning strikes or induces a transient voltage rise into utility lines, a large earth potential rise will occur at or near power poles decreasing with distance. The concrete pad that the traffic control cabinets are mounted on represents a concrete encased electrode (Ufer Ground) and will rise differently in voltage than the grounded neutral and enclosure at the disconnect switch. This will cause a significant surge current in the equipment grounding conductor from the disconnect switch to the equipment ground bar in the cabinet. The multiple neutral-ground bonds at the disconnect switch and the traffic control cabinet are causing an objectionable Continued on page 34

5 steady-state current flow in the equipment grounding conductor. During a lightning strike to the utility medium voltage distribution, re-closers will momentarily open and close causing a fluctuation of this current creating a significant voltage spike across the equipment grounding conductor (e = L di/dt). Continued from page Relocate the 30 Amp, disconnect switch from locations on or near the utility power poles and re-install in a lockable, weatherproof enclosure on the exterior of the traffic control cabinet to eliminate the effects of earth potential rise during a lightning event when the utility medium voltage distribution is struck. There was no clearly defined or adequate earth grounding technique for the traffic control system inspected, employing a combination of driven rod and buried plate at the disconnect switch. Significant lightning induced voltages can appear across even small contact impedances due to the potential lightning equalization surge currents involved. For both touch safety and sensitive equipment immunity reasons, the cabinet enclosure must be grounded by means of an appropriate sized bonding conductor. The only equipment grounding conductors for the timer and the conflict monitor are contained in the small gauge wire cable harness. Small gauge wire has significantly more resistance and inductance to the flow of high frequency currents than larger gauge wire. The fast rise time (1-10 usec) and short pulse width ( usec) of lightning surge currents represent a high frequency event resulting in larger voltage rises across conductors they transit and equipment enclosures referenced to these conductors. The bonding conductor from the light poles to the disconnect switch / traffic control cabinet earth grounding system was cut. As grounded neutral and equipment grounding conductors are delivered to each traffic light from the traffic control cabinet, it is important to control instantaneous voltage differences and resulting surge currents between them caused by differing earth potential rise during a lightning event by means of appropriate sized bonding conductors. For effective shielding and avoidance of low frequency power currents flowing in data communications shields, it is important to ground shields at one end only. There appeared to be no standard policy of grounding shields. The shield at the intersection inspected was floating. Best practice is to ground shields only at the master location. The insertion loss indicated on the specification sheet for the AC power surge protective device indicated significant attenuation beginning around 50kHz which would correspond to the standard surge current test waveshape of 8x20 usec (1 / (20 x 10-6) = 50 khz. This would suggest a filter technique dependent upon frequency of the surge current for performance in limiting voltage surges. It could not be determined from the specification sheet whether the hybrid design included metal oxide varistors or silicon avalanche suppressor diodes normally used in surge protective devices that are not frequency dependent. Recommendations The following recommendations are intended to improve performance, reduce downtime and limit storm related damage to all traffic control systems located in Hamilton, Ontario: Page Per Canadian Electrical Code [10-700(2)] install two, 3.0 meter driven rods spaced 3.0 meters apart as close as possible (each within 1.0 meter) to the traffic control cabinet. It is recommended that the two rods be bonded by means of a minimum #2 AWG, bare, stranded, tinned copper conductor, encased in conductive cement (according to manufacturer instructions), at a minimum depth of 0.5 meter below grade. The use of metal plates as earth grounding electrodes should be avoided unless rods cannot be driven due to bedrock. A National Electrical Grounding Research Project concluded that the average earth ground impedance of a vertically driven 8 x 5/8 rod was 12 Ohms and a stranded copper plate buried at 30 was 152 Ohms. 3. Install an isolated, master ground bar (MGB) inside of the traffic control cabinet. Conductors connected to the MGB should be organized according to the PANI concept with surge producing conductors connected to the P zone, absorbing earth ground electrode conductors connected to the A zone, and in this case all other grounding conductors to the non-isolated and isolated N and I zones. Bond the MGB to the closest point of the driven ground rods and associated bonding conductor by means of a minimum #2 AWG, bare, stranded, tinned copper conductor that is connected to the A zone of the MGB. 4. Bond the neutral conductor in the disconnect switch to the enclosure of the disconnect switch and to the A zone of the MGB by means of a minimum #6 AWG, green insulated, stranded copper conductor. 5. Remove the bonding conductor between the neutral bar and equipment ground bar in the traffic control cabinet. Create a minimum 0.25 clearance between the phase, neutral and equipment ground bars from the traffic control cabinet enclosure. 6. Bond the enclosure of the traffic control cabinet to the N zone of the MGB by means of a minimum #8 AWG, green insulated, stranded copper conductor. 7. Bond the equipment ground bar to the I zone of the MGB by means of a minimum #8 AWG, green insulated, stranded copper conductor. 8. Bond the enclosures of the timer and conflict monitor to the equipment ground bar by means of minimum #10 AWG, green insulated, stranded copper conductors. The PANI concept need not be followed for the equipment ground bar. 9. Bond all signal light poles together and as they are possible strike points for lightning to the P zone of the Continued on page 36

6 MGB by means of minimum #2 AWG, green insulated, stranded copper if in conduit and bare, stranded, tinned copper if direct buried. This may be done in a daisy chain, loop configuration around the intersection with both ends of the loop connected to the P zone of the MGB. Unavoidable ground loop currents are not a concern in bonding conductors that reference different earth ground potentials, as they are in equipment grounding conductors. 10. All cable shields can potentially conduct lightning equalization currents. For master traffic control cabinets, ground the shields of all data communications cables to the P zone of the MGB. Float the shields at all slave master control cabinets. 11. Replace what appears to be a filter type, frequency dependent surge protective device (SPD) for transient overvoltage and lightning surge protection of the 30 Amp, 120 Vrms, single-phase power supply with a nonfrequency dependent SPD. Silicon avalanche suppressor diode (SASD) technology is recommended for its fast response time, low voltage clamping and non-degrading characteristics. The SPD should be fused and wired according to the recommendation of the manufacturer with leads as short as possible to the point of protection. Ground the SPD to the N-G bond in the disconnect switch or the P zone of the MGB depending upon point of application. Continued from page 34 was to keep transient energy away from and out of the controller. Unfortunately, this results in a grounding topology which is inferior from the perspective of step and touch potentials as well as electrostatic discharge. Furthermore, the function of the power supply SPD located inside the controller cabinet will be seriously compromised due to the long, high impedance path to earth via the service bond and ground connections. Finally, this topology fails to consider that transients will also enter the controller on the communications and signal cables and the SPD protecting these conductors will have no local, low impedance grounding path to earth. It is a common fallacy that electricity follows the path of least resistance, rather and in accordance with Kirchhoff s law, it follows all available paths in proportion to their relative impedances. Unless a perfect Faraday shield can be established, transient energy can always find its way into any control cabinet through various coupling mechanisms such as conduction and induction - on any wire or cable going in or out. The best protection strategy is to deal with it by establishing a bonded, equipotential earth reference with a single point, low impedance path to ground using the optimum termination sequence known as PANI. This approach will assure improved safety, efficient SPD function and the best possible protection and operation of sensitive electronic devices. 12. Ground the data communications SPD to the P zone of the MGB. Corrective Actions Since this report was filed by RO Associates, the City of Hamilton has completed four major tasks (1) the development of a grounding method using only a single rod buried in ground enhancement material which yields 5 ohms to earth; this is significant for retrofit applications where restricted surface area and the proximity to underground utilities in a dense urban environment limits the maximum size of a grounding array and the number of grounding electrodes that can be deployed (2) a draft version of our new grounding and bonding standards for traffic signal equipment is now in place for the construction of new signalized intersections (3) a custom, service entrance panel has been designed which will employ a high quality SPD and superior grounding and bonding terminal facilities (4) a stray voltage audit has been conducted to assess traffic signal and street-lighting hazards and their relationship to grounding and bonding practices. In 2010 a contractor will be hired to begin repairing and upgrading the existing traffic grounding and bonding infrastructure to comply with our new standards. Historical Note on Grounding Topology and Design Philosophy: Ontario Provincial Standards for grounding traffic signal controllers were based upon the philosophy of redirecting transient energy into the earth near the electrical service panel (which is usually mounted on a utility pole) and then locating the traffic signal control electronics far away from that point - with no local ground reference. The intent Page 36 The City of Hamilton endorses the design philosophy espoused by RO Associates which is in compliance with IEEE recommendations found in their Green and Emerald standards. Furthermore, the City of Hamilton no longer endorses or permits double neutral-ground bonding (at both the service entrance panel and in the traffic signal controller). Alan Rebeck Mr. Rebeck, Director of Engineering for RO Associates, a Smiths Interconnect Protection Technology Group company, holds two Degrees from Rutgers University, including a B.S. in Electrical Engineering. He is a member of the IEEE Power Engineering Society Standards Committee for Surge Protection, the 2008 NEC Task Force on grounding and bonding, the Lightning Protection Institute as well as a U.S. representative to the IEC Surge Protective Devices Standards Subcommittee. He is a regularly featured speaker to many technical associations and conventions throughout the world including the International Municipal Signal Association (IMSA) and has performed more than 300 power, grounding and lightning protection surveys at telecommunication sites and manufacturing plants in more than 30 countries. Daniel Lawlor Mr. Lawlor, Electrical Project Manager for the City of Hamilton, Traffic Engineering Section has an Electronics Engineering Technologist and a licensed electrician. He as over 25 years of hands on experience in the electrical and electronics field and is a member of the Ontario chapter of the IMSA. In 2008 he completed the Ground Systems Design and Testing course at the AVO Training Institute in Dallas, Texas. He has conducted extensive research into the subject of traffic signal equipment grounding and has developed new standards for the City of Hamilton. For course and consulting information, contact , bheinig@roassociates.com or visit com.