Effectiveness of Equipotential Bonding Mats

for Hydro-excavation University of Alberta researchers: Ming Dong, Shengqiang Li, Bing Xia

Background & Scope • In recent years, the effectiveness of the equipotential bonding mat has

become an industry wide concern (e.g. Kinectrics report and CEATI project)

• Some APIC companies such as EPCOR have specific concerns on the use of the bonding mat under certain working conditions (such as hydro-vac job)

• This project is conducted to provide scientific information so that APIC companies can make an informed decision if they want to continue or to revise work practices which involve the use of the bonding mat.

Industry collaborators: Dennis Rasmusson, Harvey Sypkes, Altalink; Clark Laing, ATCO Electric; Ray Cislo, Richard Vercholuk, Epcor; John Price, Enmax; Michael Simone, Philip Laing, Fortis Alberta

EPCOR’s technician using equipotential bonding mat during hydro-vac work

Scope of this investigation:

• The portable bonding mats that are similar to those marketed by KriTech

and Hubbell;

• The mat that is laid on the ground surface when being used;

• The worksites that could be energized by distribution facilities;

• The worksites that are outside substation.

Investigation strategy • Establish an ideal mat to clarify the issues and requirements;

• Compare the actual mat with the ideal mat to identify differences and

deficiencies;

• Determine if the differences/deficiencies are acceptable from a practical perspective;

• Conservative assumptions are used in this study since the mat is expected to work under various, unpredictable worksite conditions.

Ideal bonding mat (1)

An ideal equipotential bonding mat is a (solid) conductive plate that meets the following requirements:

Vtouch

Vstep

Bonding cable

Cable for trip groundingIdeal mat

(Metal plate) (a separate practice)

( )

( )

touch touch safety limit

step step safety limit

V V Duration

V V Duration

Requirement 1: Current Carrying Capability The mat is required to carry the maximum fault current expected for the worksites it is intended for. Since the mat has a resistance to ground (Rmat), it will limit the fault current. This situation is depicted as below.

Bonding cableIF

Rmat

ZsystemIF

Rmat is the mat-to-

ground resistance.

The estimated value is

4ohm to 200ohm

IF

Vsystem

5 kA 8 kA 20 kA 25 kA 30 kA

5 kV 0.189 0.190 0.192 0.192 0.192

15 kV 0.546 0.558 0.570 0.571 0.572

25 kV 0.875 0.908 0.941 0.945 0.948

Fault current (in kA) of different systems with Rmat=15 ohms

Rated If

Rated Voltage

Ideal bonding mat (2) Requirement 2: Resistance of the Bonding Mat The resistance of the mat must be small enough to meet Vtouch and Vstep limits. According to IEEE std. 80, the withstand current for a 70 kg human body for a fault duration of TSC is:

0.157d

body F

B d SC

RI I Amps

R R T

Vtouch

Bonding cable

Imat

Ibody

Wet spot

C1

C2 Vtouch

Bonding cable

Imat

Ibody

Wet spotVtouch

Bonding cable

Imat

Ibody

Wet spot

C1

C2

RB

IF

Rd H (C1)C2Imat

Touch Voltage

Ibody

(a) Scenario diagram (b) Equivalent circuit

Tsc

=10

Tsc

=20

Tsc

=40

Tsc

=60

cycles

cycles

cycles

cycles

Tsc

=5 cycles

0 1 2 3 4 5 6 7 8 9 100

100

200

300

400

500

Actual IF

(kA)

Re

qu

ire

d R

d(m

)

Tsc

=120 cycles

Worst scenario of touch voltage on ideal mat

Required diagonal resistance Rd (mΩ)

Requirement 3&4: Size and Connection:

Earth Surface

PotentialDistance

Potential of the

Metal Plate

Ideal Mat

Vbody

Earth Surface

Potential

Step voltage hazard at the edge of the ideal mat

Size must be large enough to avoid accidental stepping out.

HV Power

Source

Ileak Ileak Ileak

Ileak IleakIleak

HV Power

Source

Higher-than-normal

step voltage!Ileak Ileak Ileak

2Ileak 3IleakIleak

Desirable Undesirable

The mats shall be connected in a way to avoid large current flowing through them.

Additional protection-Grounding the mat:

Vtouch

Vstep

Bonding cable

Vtouch

Vstep

Bonding cable

Vtouch

Vstep

Bonding cable

Ground

CableVtouch

Vstep

Vtouch

Vstep

Vtouch

Vstep

Ground

Cable

Attach to the working object Attach to the corner of the mat

Fast trip of fault; Divert fault current from mat; Reduce ground potential rise

Additional protection-Isulation layer on top:

Increases the body resistance; helps to reduce body current. Useful to improve safety.