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Publication Number: FHWA-HRT-06-139
Date: October 2006

Traffic Detector Handbook:Third Edition—Volume II

APPENDIX O. GROUNDING (MAINTENANCE GUIDELINES)

SECTION I—TRAFFIC SIGNAL GROUNDING

1. HYDRO GROUNDS

(1)		Inadequate hydro grounds at a service pole will allow energy from lightning discharge into the controller. Inspect Hydro grounds visually and arrange for the Supply Authority to repair faulty grounds.
(2)		At controller sites where lightning problems have occurred, the Hydro ground should be reinforced to two 20 x 3000 mm copperclad rods at 3 m spacing at the service pole and at one hydro pole each way. Arrange purchase order for this work if necessary.
(3)		Do not use Hydro ground as a Service ground.

2. SERVICE GROUNDS

(1)		To reduce voltages transferred from the Hydro ground (under lightning discharge) to the Service ground, the Service ground should be located as far from the Hydro ground as possible. See Figure O-1.
(2)		The service ground should consist of a minimum of four 20 x 3000 mm copperclad ground rods on #2/0 bare copper wire grid as per Figure O-1.

Figure O-1 shows controller cabinet with four 3-meter (9.8-foot) ground rods, spaced 3 meters (9.8 feet) apart in the shape of a square, at a minimum distance of 5.5 meters (18.0 feet) from the controller cabinet. In addition, it shows 2 hydro ground rods and a hydro ground wire at a minimum distance of 11 meters (36.1 feet) from the cabinet. The hydro ground wire is shown ascending the supply pole. The ground wire is shown as number 5 RWU990 ground wire in rigid steel duct with feeders. The supply service is shown in 25-millimeter (0.98-inch) rigid conduit with number 2/0 grounding.

Figure O-1. Recommended improvement to system ground connection.

3. CABINET GROUNDS

(1)		Proper ground bus bar should be installed in the controller cabinet and all ground leads from equipment should connect to this one point.
(2)		The controller cabinet ground bus should be connected to the service solid neutral by a #6AWG RWU90 insulated system ground wire.

4. SPECIAL SOIL CONDITIONS

(1)

Grounding system in areas of sand, gravel, or rock may require reinforcement due to poor soil conductivity. Refer to Electrical Design Manual—Volume 1. Request design aid from Electrical Engineering Section if necessary.

5. SYSTEM GROUND

(1)		System ground wire for signals to be #16 AWG RWU90—40°C green.
(2)		Insta11 connections of the three systems (Service ground, cabinet ground, and equipment ground) as indicated in Figure O-2.
(3)		Ensure that all metal enclosures are bonded to the proper section of the system ground.
(4)		All connections to be thermit weld, impact or compression type (no split bolts, etc.).

6. ROUTINE INSPECTION

(1)		Yearly inspection frequency for:
	(a)	Tight connections of accessible ground wires (controller cabinet and service); corrosion.
	(b)	Hydro and service grounds; visual for obvious damage from vehicles, etc.
	(c)	Testing of resistance to ground and soil conductivity (service ground) as per Subsection 8.

7. EMERGENCY INSPECTION

(1)		For controllers damaged by lightning:
	(a)	Inspection Hydro ground as per Subsection 1.
	(b)	Have Supply Authority check distribution arrestor and upgrade ground and arrestor if required.
	(c)	Inspect for proper installation of grounds as per Subsections 1 to 5.

In the first diagram, figure O-2 shows a pictorial representation of the signal head mounting poles at an intersection. Each pole is shown linked to equipment ground. Grounding wires are shown going around the intersection and to a pulling point. The ground is then connected to the service cabinet and then to the hydro ground. The lower diagram shows the same layout but as an electrical wiring layout. In the latter picture, it is more clearly show that the lightning arrester will cause surges to be bypassed to the hydro ground.

Figure O-2. Signal grounding system (with or without lighting). No ground electrodes at cabinet.

8. MEASUREMENTS

(1)		Resistance to ground and soil conductivity should be measured at an ’average’ time of year. Measurements taken when soil is wet or frozen are not meaningful.
(2)		Use the Megger ground resistance meter and the two small ground rods supplied with the unit. Follow instructions exactly for distances or results will be meaningless. For information regarding the instrument, contact the Electrical Operations Unit.
(3)		Measurements to ground to be taken at the SN of the service and the controller cabinet ground bus.
(4)		Resistance to ground and soil resistivity are related mathematically. See Electrical Design Manual—Volume 1, Section II.
(5)		Resistance to ground should be 10 Ω or less but can vary up to 25 Ω in high resistivity soils. Readings over 25 O indicate that further inspection and repairs or replacement should be done or that additional rods and wire should be added (new installations). Add additional elements as per Table O-1.

9. STEEL FOOTINGS

(1)	Steel footings may be considered the equivalent of a ground rod for resistance to ground purposes.

10. GROUND RODS

(1)	One rod length spacing to be obtained between rods.
(2)	Connecting wire is more effective if buried to 1/2 the normal frost depth (assuming snow cover).
(3)	Do not install rods at more than 45° angle. Driving jaws for use with a hydraulic drive head are available.
(4)	Use 20 x 3000 mm copperclad rods only, with thermit weld or impact connectors.

**Table O-1. Resistance to ground for various ground system configurations and soils.**
Description	Normal Use	Resistance to Ground(Ω) in Clay (ρ=100Ωm)	Resistance to Ground(Ω) in Sandy Clay (ρ=200Ωm)	Resistance to Ground(Ω) in clayey sand (ρ=500Ωm)	Resistance to Ground(Ω) in sand (ρ=1500Ωm)	Resistance to Ground(Ω) in sand, Gravel(ρ=5000Ωm)
1. Single 20 mm x 3 m rod	Addition to system	32	80	160	480	1610
2. Single 220 mm dia. x 2300 mm steel footing	Poles (requires additional system)	28	70	140	420	1400
3. Single 85 mm dia. x 1830 mm steel footing	Poles, cabinets (requires additional system)	40	100	200	600	2000
4. 610 x 610 x 6 mm plate	Rock overburden 0.6 to 2.0 m	46	115	230	690	2300
5. Single #6 wire, bare, 3 m long	Addition to system	41	103	205	615	2050
6. Single #2/0 wire, bare, 3 m	long Addition to system	38	95	190	570	1900
7. Single #6 wire, 2 rods	Service	19	38	95	290	950
8. Single #2/0 wire, 2 plates	Service in overburden	27	54	140	410	1400
9. 220 mm dia. x 2300 mm steel footing, #6 wire, 1 rod	Poles	19	38	95	285	950
10. 85 mm dia. X 1830 mm steel footing, #6 wire, 1 rod	Poles	16	34	80	240	800
11. 85 mm dia. x 1830 mm steel footing, #2/0 wire, 2 rods	Cabinets	14	28	70	210	700
12. 85 mm dia. x 1830 mm steel footing, #2/0 wire, 3 rods	Cabinets	13	26	65	195	650
13. 85 mm dia. x 1830 mm steel footing, #2/0 wire, 4 rods	Cabinets	10	20	50	150	250
14. #2/0 wire, 4 rods	Service Any for ρ < 125 Om	11	22	55	165	550
15. #2/0 wire, 4 rods, 2 ties	Any for ρ < 125 Om	11	22	55	165	550
16. #2/0 wire, 4 rods, 2 ties, 4 tails	Any for 125 < ρ < 150 Om	(9)	18	45	135	450
17. #2/0 wire, 4 rods, 2 ties, 8 tails	Any for 150 < ρ < 200 Om	(6)	12	30	90	300
18. #2/0 wire, 8 rods, 6 ties	Any for 200 < ρ < 350 Om	(5)	10	25	75	250

11. GOOD PRACTICE

(1)		Grounding is a safety device and many members of the electrical industry are somewhat lax about proper grounding practices as the only times they are required are under abnormal conditions, such as short circuits and lightning surges. Poor workmanship or practices are not apparent until such an abnormal problem occurs.
(2)		The Ministry’s grounding system practices should meet or exceed the requirements of the Code. (The Code is a minimum requirement). If doubt exists as to practice, the regulations contained in the Code should overrule other opinions.
(3)		Workers should endeavor to follow the Ministry’s practices faithfully so as not to endanger themselves or the Ministry to the possibility of legal prosecution due to mishap through poor practice or application.

SECTION II—REFERENCES

(1)

"Equipment, Manuals and Procedures Evaluation for the Design and Maintenance of Traffic Signal Systems,"Report No. 2, Grounding. Ministry of Transportation of Ontario, May 1988.

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FHWA-HRT-06-139

Page Owner: Office of Research, Development, and Technology, Office of Operations, RDT

Keywords: Traffic detectors, sensors, detector installation, detector maintenance, signalized intersections, intersection safety, intersection treatments, infrared sensor, inductive loop, magnetometer, video image processor, microwave radar sensor, laser radar sensor, acoustic sensor, ultrasonic sensor, magnetic sensor
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