Paul Stride - Electrical earth safety

Electrical Earth Safety

24th Electrical Engineering Safety Seminar Paul Stride November 2014

Schneider Electric 2 – 24th Electrical Engineering Safety Seminar – Nov 2014 – Electrical Earth Safety

● It has been reported on more than one site that various trades people engaged in work around cabling are receiving what appears to be electric ‘tingles’.

● In most instances, no source can be confirmed after extensive testing - however in a few cases power has been confirmed at some point.

● Often, follow-up investigations have been unable to duplicate or locate the source.

●Why is it so?

Electrical Networks


‘Noise’ by any other name....

Harmonics dv/dt

EMI Conducted

EMI Radiated

Non – Linear Load

(SS / VSD / UPS etc)

Power Cable (in)

e.g. Motor Cable (out)

EMI Conducted


Harmonics

● All these devises have non-linear front ends. That is, they do not draw current off the supply with a current waveform that is the same shape as the voltage waveform. The diode front end acts like a switch so that when the AC voltage exceeds the internal DC bus, current is conducted in a LUMPY grab from the supply.

U

I U

I

ϕ


Harmonics

● Full wave rectified waveform DC bus ripple waveform.

● Line current with a weak or soft supply i.e. low impedance.

● Line voltage with distortion shown due to non linear current load.

● Line current with a strong or hard supply i.e. high impedance.

● The 5th harmonic which lines up with, and is formed as a result of, the voltage distortion.


RFI

● EMI / RFI (Electromagnetic Interference / Radio Frequency Interference) ● EMC = Electromagnetic Conformance or Compliance

● RFI starts at 50Hz2 or 2,500Hz or 2.5 kHz ● In the USA it starts at 60Hz2 or 3,000Hz or 3.6 kHz ● In Australia the Radio Frequency spectrum is managed above 3.0kHz

● RFI can be Conducted or Radiated ● Conducted as in a cable going from a transmitter to an antenna ● Radiated as in radio waves travelling to/from an antenna

● Emissions standards ● Limited to very low levels, the amount of electrical ‘noise’ a product can transmit when

powered up (without connection)

● Immunity standards ● Limited to very high levels, the amount of electrical ‘noise’ a product can withstand without

effect when performing its assigned task


Electrical Networks

● Personal protection historically relies on single primary star and/or bonded earth cables connected to a single point of reference to which all voltages and currents for the purpose of tripping can be monitored.

● To better understand this lets look at:

● the earthing arrangement ● cable characteristics

● Typical TN system

N

Local earth bar

Neutral bar

MEN Link + CT MEN trip

unit

MAIN EARTH BAR

Earths Link

E

E

T/F Earth


dv/dt

● Short cables = less capacitance + less inductance therefore less dv/dt.

● Usually limited to ~2 times the DC bus level (but can be 3 times)

● At 415vac, DC bus = 586.9vdc therefore dv/dt is typically ~1173.8vp

● At 440vac, DC bus = 622.3vdc therefore dv/dt is typically ~1244.5vp

● dv/dt is recoded as a function of slope and is typically between 2 to 10kV/µS

HEA

T

HEA

T

SHO

RT

CC

T

OPE

N C

CT

OPE

N C

CT

HEA

T

HEA

T

SHO

RT

CC

T

OPE

N C

CT

OPE

N C

CT


dv/dt

● Set oscilloscope to trigger off the PWM out voltage spikes ● Increase slightly until the trace is no longer triggered ● Turn off out of the drive ● Start the drive output and capture the very first scan. ● Subtract the upper and lower 10% of the voltage ● Measure the time between the resultant

10% to 90% straight line graph. ● The dv/dt is at its worse

when the cables and motor magnetising currents are zero, i.e., on the first turn-on cycle.

dv

dt

10%

90%


Electrical Networks

● In the worlds quest for lower cost consumables and greater process efficiencies, ● the switch mode power supply, ● three phase rectifier, ● LED lighting,

● all played their part but what of the consequences when it comes to safety?

● Typical IT system

Local earth bar

MAIN EARTH BAR

Earths Link

E

E

T/F Earth R

N Neutral bar


What’s Hasn’t Changed?

● Heat = Lost Energy Less heat = smaller device = less cooling = less moving parts = increased reliability

● When this switch is open – how much heat is generated?

10 amp cable 10 amp switch

10 amp cable

0 amp


What’s Hasn’t Changed?

● Heat = Lost Energy Less heat = smaller device = less cooling = less moving parts = increased reliability

● When this switch is closed – how much heat is generated?

10 amp cable 10 amp switch

10 amp cable

1 amp


Resistance v Impedance

● Please note that the scales that are typically shown for a cable are non-linear in both the X axis and the Y axis

● For a 35mm earth cable on an MCB we can see the resistance over 20 metres is 20 x 0.5mΩ @ 50Hz = 0.01 Ω

● For a 35mm earth cable on an MCB we can see the resistance over 20 metres is 20 x 0.1Ω @ 16kHz = 2 Ω


Radius Diameter Area Circmfrnc Strands Area Check Total Circmfrnc

2.25700 4.51400 16.00987 14.18686 1.00000 16.010 14.187

0.85286 1.70571 2.28600 5.36081 7.000 16.002 37.52567

0.19943 0.39886 0.12500 1.25357 128.000 16.00000 160.456

25.53963 51.07926 2050.00000 160.53482 1.00000 2050.000 160.535

Resistance v Impedance


Parallel Circuits

● If a parallel path is offered to a voltage the current will divide in the opposite relationship to resistance.

● The total circuit impedance is 2 Ω ● The cable impedance is 0.00995 Ω with a majority of the current taking the

path of least resistance – for the nominated frequency!

10V Z = 0.01 Ω

Z = 2 Ω 5 amps

Load

ZT = 0.00995 Ω

I = 4.975 A

I = 0.024 A

50Hz


Parallel Circuits

● Impedance is effected by frequency. ● It is possible for the impedance of the two parallel paths to ‘swap’. In one

cable the impedance may decrease as the frequency increases and the other may increase in impedance.

● Usually one path increases slowly while the other increases dramatically. ● The total cct. impedance is now 2.5 Ω ● If the load remained linear at

1.99005 Ω then the parallel cable impedance has increased from 0.00995 Ω to 0.50995 Ω

● This results in there now being a 2.0398 volt drop across the parallel cable path

10V

4 amps

Load

ZT = 0.50995 Ω

500Hz

VD = 2.0398 V


Refraction & Reflection

● When a frequency, higher than the fundamental, travels down a cable, just like light, if there is a change in media density, some energy is reflected back, some energy ‘bends’ to another frequency and in doing so a small amount of heat is produced in most cases.

● At 50Hz these values are almost immeasurable and definitely insignificant however as we move up the frequency spectrum then these figures start to become significant.

● This is particularly important when we look at submersible motors with motor tails supplied, long runs in petro- chemical plants or retro-fits to existing plants in part using existing infrastructure.

● We may change cable type more than once.


Voltage Interaction

● Wave interference is the phenomenon which occurs when two waves meet while travelling along the same medium. The interference of waves causes the medium to take on a shape which results from the net effect of the two individual waves upon the particles of the medium.

● This means that if a voltage waveform of 850Hz (17th Harmonic) is travelling left to right and meets a reflected wave of 1550Hz (31st Harmonic) travelling in the opposite direction, at the crossover point the voltage is the sum of the two voltages. That is the 850Hz with the 1550Hz transposed on top of it as measured at a single point in the cable.

Voltage A + Voltage B

A B


Practical Application

PLC

I / O

PLC

I / O

Electrical Switchboard

35mm2 35mm2

30mtrs 30mtrs


Practical Application

PLC

I / O

PLC

I / O

Electrical Switchboard

35mm2 35mm2

30mtrs 30mtrs

VSD’s VSD’s ?

Oscilloscope


Capacitive Coupling

● Dependant on normal capacitive impedance values. ● Length and width of parallel plates (surface area) ● Dielectric Material ● Distance between plates (cable insulation thickness) ● Frequency of applied signal

= Capacitive reactance

Xc = (2πFC)-1

Such that the higher the frequency, the lower the impedance

=

Insulated Cables Red Blue


Inductive Coupling

● Dependant on normal inductive impedance values. ● Current (magnetic field strength) ● Distance between conductors ● Frequency of applied signal

= =

Insulated Cables Red Blue

Inductive reactance XL = 2πFL

Such that the higher the frequency, the higher the conducted impedance


What is the real potential? ● Voltage, Frequency, Current and Time

● Voltage - Although many variations exist, a plasma lamp is usually a clear glass

sphere filled with a mixture of various gases (most commonly neon, sometimes with other noble gases such as argon, xenon and krypton) at below (1/10th) or approaching atmospheric pressure. They are driven by high-frequency alternating current energy, typically via a fly-back transformer, with between 15 - 35 kHz at around 2 to 10kV.

● The lower atmospheric pressure inside allows a longer free path for the charged particle carrier before a collision. The longer the free path the greater the kinetic energy but more importantly the lower the voltage required for the same effect.

● When the hand comes in contact with the surface an additional earth plane is introduced and the charge takes the path of least resistance, travelling over the skin surface to earth.

● Why doesn't it kill people?



● Frequency - Conventional Transcutaneous Electrical Nerve Stimulation (TENS)

and Electrical Muscle Stimulation (EMS) machines deliver electrical pulses at low frequency of 2-150 pps via sticky pads on the muscle tissue for the purpose of developing muscle tissue, improving circulation and blocking pain.

● Sometimes these use two medium frequencies that beat against each other between 8-10kHz at between 40 - 80 volts. The higher the frequency the less skin resistance and the higher the current capability.

● As the pulse frequency is adjustable between 2 and 150Hz, including the dangerous 50Hz range why are these devices even legal?

● Why doesn't it kill people?



● Current – While not the whole story, this chart, still in use in some jurisdictions, nominates 10mA as “Severe” and 100mA as deadly with no mention of voltage at all. Why then would our household RCD be designed to allow over 10mA?



● Time - It is often considered the maximum duration in Seconds = 0.116/(V/R) that is for a 240vac circuit, with a body resistance of 1000ohm the maximum contact time is 483mS.

● Therefore if a 80 volt static discharge is dissipated in 10mS at a current of 60mA then no damage may occur.

● The question is in some way, how much energy in kJ will the body experience? Is it measureable and repeatable? What external influences can effect it?



● In this diagram from a DPI document we can see that a decrease in voltage results in an increase in time allowed.


Electrical Networks ● Very Simplified Conceptual Equivalent Diagram on a TN Network

Harmonics from VSD

Cable Impedance

Busbar Impedance

Supply T/F

VSD PWM

Impedance Structural Resistance

Impedance Earth Resistance

Capacitive & Inductive coupled

Motor dv/dt




N




Cable & connection impedance

3

http://www.pdclipart.org/displayimage.php?album=141&pos=112













Electrical Networks ● A bit more closely: ● Potentially - at 415vac,

DC bus = 586.9vdc VSD Motor

PWM Voltage at VSD output

60vac

30vac

1vac

0.2vac

0.10vac

0.25vac

100mvac

200mvac Connection Impedance

Conductor Impedance

Connection Impedance

0.0vac


Conductor Impedance


Peak = up to 3 times DC bus voltage.











PWM Voltage at VSD output

60Vac

30Vac

1Vac

0.2Vac

0.10Vac

0.25Vac

100mVac

200mVac Connection Impedance

Conductor Impedance


0.0Vac


Conductor Impedance


Human Resistance is in parallel with network

20mA

20mA












PWM Voltage at motor

120vac

50vac

1vac

0.2vac

0.10vac

0.25vac

100mvac

200mvac Connection Impedance

Conductor Impedance


0.0vac


Conductor Impedance


Human Resistance is in parallel with network

30mA

10mA










Electrical Networks ● Simplified Conceptual Equivalent Diagram on a IT Network

Harmonics from VSD

Cable Impedance

Busbar Impedance

Supply T/F

VSD PWM




Motor dv/dt




N




Cable & connection impedance

3

IT resistor















Influences - environment

● Its all about the: ● Finding the source of the energy

● The number and value of alternate paths offered

● The impedance influencers such as humidity, physical movement, maintenance

practices etc.


Earth Leakage Currents

RFI filter Differential breaker

bearings

Low frequency leakage current (generated by the EMC filter)

High frequency leakage current (generated principally by the stray capacitance)


Recommendation ● Determine the source of the potential problem.

● Apply the appropriate fix for the problem.

● Usually putting in a balanced earth, shielded cable would be an adequate solution. Doing so when the earthing network is inadequate even if only at the high frequency spectrum, is potential for disaster.

● A perfect solution would be to replace all cables associated with non linear loads such as VSD’s, UPS’s, lighting, switch mode power supplies etc, with a shielded cable striping back to the shield every 1-2 metres and bonding to a separate earth stake with high strand count cable will in all likelihood remedy the situation.

● If not fixed, the solution could be to include all metal structures that potentially could act as antenna’s with a similar earthing network, but then at what cost?


Finding the Source

● “What is the frequency?” is usually a good clue.

● For VSD sourced noise, change the PWM base frequency and determine if the earth frequency changes. It may not reduce the voltage or current but it can help identify the source.

● Check associated earths to/from the ‘source’ for high frequency impedance and repair/replace as necessary. Reducing the quantity of the high frequency in the PWM will reduce the current in the capacitive coupling to earth only if the frequency is in this spectrum. This can be done with a motor choke or motor filter. Line filters addressing harmonics at the front of VSD are unlikely to address the frequency spectrum above 3kHz.

● Be aware that lighting systems such as “fluro’s”, including some new LED and plasma lighting systems can produce significant high frequency noise.


The Source

● Was the current continuous or is it a dc static discharge?

● Is the discharge caused by conducted energy into the area or energy radiated into the area?

● In some installations the solution may be to install insulated cable tray while in other situations, wrapping the whole system in a giant conductive earthed tray is the solution.

● Through correct measurement, it may be discovered the initial contact was static generated through rotating machinery or rotating belt and the residual is conductive, with the residual proving to be relatively low and at a high frequency therefore posing no risk at all.

● When to use high strand count flexible cable and when NOT to use it.

● When to use shielded cable and when not to use shielded cable.


Reduction Through Design

● Knowing when frequency combine and when they are attenuated is an important aspect of network design.

● Often power consumers at one frequency can become a suppler at another frequency.

● Distance (attenuation) is important between transformers and AHF’s and PFC’s in most cases with regard to the high frequencies.

PCC2

PCC1

PFC

AHF

A B C D E F G H J

Cho

ke

Line

C

hoke

Line

C

hoke

Line

C

hoke

Line

C

hoke

Line

C

hoke

Line

C

hoke

Line

C

hoke

Line

C

hoke

UPS

AFE VSD

Choke


Resources

● Schneider’s own Dr. Shuli Jao. Currently our Lead Energy Consultant as well as an expert in the design of instrumentation used to capture, measure and analyse these types of problems. Shuli has conducted extensive worked and studied in Europe as well here in Australia through Charles Darwin and Western Sydney Universities.

● Schneider's own Raed Odeh. Currently studying at the University of Wollongong, Raed is part of the Schneider Edison programme. This programme serves to identify exceptional engineers and place them into world wide, multi-disciplined think tank with other Schneider and non-Schneider partner companies around the world.

● Peter Stepien (PhD, BE (Hon 1), MIEEE, MIEAust) of in Newcastle has done some very impressive work in this area and along with colleagues have some amazing ‘toys’. Attached to the University of Newcastle, and in collaboration with Ampcontrol, Peter continues to show that Australia’s knowledge and expertise in this field is as a world leader. http://www.restech.net.au/ Ph: (02) 4033 9155

● The University of Wollongong have produced some excellent articles and have numerous research fellows working in this area of power quality and disturbances. They continue to challenge, inspire and excel in this field.

http://www.restech.net.au/


Final Say

●Measure it accurately, ●Determine the source of all the components, ●Eliminate, contain or isolate the problem in the

correct sequence as close to the source as possible.

●The End

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Paul Stride - Electrical earth safety

Government & Nonprofit