Transient Properties of Conductors

Welcome to IBEX session 106: Handling High Intermittent Current Loads

Presented by: Wayne Kelsoe, Blue Sea Systems, Ed Sherman, American Boat & Yacht Council

1Considered as transient events, the level and duration need to be considered..In the case of fuses and circuit breakers, think in terms of trip delay curves2Graph Y axis shows time, X shows multiples of constant current

3Outcomes will vary depending upon the type of event.Is it a current overload due to operational norms? Motor start up for example.

Is it a sharp and immediate current spike due to a short circuit?

The type of event will determine reaction time and long term impacts on equipment. 4When all is said and done, overcurrent protection devices must be able to deal with:Short circuit currents, at potentially very high levelsInadvertent grounding contact, insulation failure, chafe, inadvertent mechanical contact while servicing or loose, unsecured equipment in engine room spaces.Overload currentsMotor Overloads, locked rotorsLoad additions ultimately exceeding panel board feedersAssumptions of load statistics are exceeded.

5The term transient event needs some clarification perhaps..For our purposes, it is an event of short enough duration that the system does not reach equilibrium.6What are the practical applications and concerns?Explains level of heating from events like engine starting, thruster operation or motor starting.May help us choose fusing if we want to fuse starting circuits.Provides part of the criteria for sizing grounding wires under NEC, which may simplify installations if adopted by ABYC.Provides a way to size battery switches that deal with a mix of loads, and starting currents.7OK, so what about the transient properties of conductors?The simple question is: What happens to on board wiring when exposed to these transient current?In the photo here, we have someone trying to test out not only transient, but possibly continuous DC load affects on wiring

8Typical Response to a step up in current:9All conductors follow this shape but:The steepness at the start of the rise is determined by heat in and mass. (Cross section)

The time constant depends on the size of the wireIt is proportional to the diameter

The final top of the curve is determined by resistance (losses) and surface area (cooling) and equals the steady state rating.

10Wire ratings are not constant A/area!If you plot out the wire tables, ampacity does not grow as fast as wire cross section but about to the power.

The transient capacity (near the bottom of the curve) is proportional to cross sectional area.

That means that the ability to absorb transients is greater (compared to steady state rating) as wire goes up in size1112Typical Response to a step up in current:13We can use the same math to plot current to achieve a fixed temperature rise:14Notice that the current descends as time increases.For really brief events, we can carry much more than the wire rating.

For a our 0 wire, we can handle over 1000A for 30 seconds of cranking without overheating the wire!

Thats not easy to see, so lets blow it up a bit15Expanding out the short term results:16We have to be cautious using this information1. Voltage drop considerations may dictate our wire size, not thermal considerations.

2. We seldom can match circuit protection time delay curves to the time delay curves of larger wire.

Fuses will tend to overprotect in the short termorWill under protect in the long term

1718May be useful in Special CasesSome loads are short term by their nature:Starters< 30 seconds (according to SAE)Windlasses< few minutes if properly usedThrusters< few minutes if properly used

Starter circuits, by rule, are not required to be fused.Windlass and Thruster manufacturers may encourage selecting wire sizes by voltage drop. Short wires may be smaller than we would expect or ABYC tables allow. 19StartersThe reasoning behind not fusing starters is that starting batteries didnt have much more than the capacity for starting and cant feed excessive energy into faults.The risk from a fault in a starter circuit is less than the risk of not being able to start after a nuisance fuse blowing incident.

Things have changed! We regularly supply cross connects from massive inverter/house banks to the starter circuit. That whole path is seldom fused because of this rule.20Any fuse is safer than no fuse!A fuse in the starter circuit should be sized to avoid nuisance blows.Avoid excursions far over the fuse rating, even though under the delay curve because fuse fatigue can occur from repeated stress.Some fuses achieve delay by an alloying action as the fuse heats, but this is irreversible and cumulative resulting in potentially blowing after 20 or 100 starts. (Class Ts and ANLs are not alloy fuses, however)The wire capacity under E-11 Table VI is a poor guide in this application because of the short term.

21Fuse Selection for startersRecommended Fuse Size:Fusing at 100-200% of the motor cranking current should still protect against external low Z faults and locked rotors. NEC has determined grounding wire sizes sufficient to mate with large scale fuses in Table 250.122. This table allows fusing at 2X wire rating (#4) up to 5X wire rating (4-0)Minimum wire size:The engine manufacturer will normally recommend a wire size.Wire should have at least a minimum capacity at 1 minute equal to the engine cranking rating regardless of how short it is.Battery switches are tested and achieve their ratings with specific wire size. If the starting current is near the switch starting rating, the switch manufacturers rating may dictate the minimum wire size.

ABYC requires a battery switch as a manual backup in the event of a stuck starter solenoid. We could have a fault not of excess current, but of excess time.

22Wire transient ratings based on time constants:105CTime in MinutesAWGDiameterTime Constant VI-A 0.5135inchesMinutesAmps180.04031.242034.726.921.020.2160.05081.562547.736.327.125.5140.06411.973573.955.539.636.5120.08082.4845105.378.153.748.3100.10193.1360156.1114.776.467.280.12853.9580231.9169.1109.694.460.1624.97120388.0281.2178.3150.740.20436.27160578.0416.8259.5215.930.22837.01180686.1493.7305.1252.120.25767.91210848.4609.3373.8306.810.28938.882451047.2750.8457.6373.400.32499.982851288.9922.8559.2453.92-00.364811.203301579.31129.1680.8549.93-00.409612.583851950.01392.5835.7672.24-00.4614.124452386.01702.01017.2815.023Thrusters and WindlassesA major manufacturer recommends wire sizing based on voltage drop. Voltage drop helps reduce inrush current and increase life of contactors, so they dont want a very low drop circuit.Willing to accept some loss in output and lower speed to get the contact life.Operating time is in minutes, this manufacturer rates the motor for three minutes of run time24Wire Table from a Thruster ManufacturerTT ModelCurrentAWG (Circuit loop Length in feet)Amps32100000000002x0000140 2.0KW-12V27020 ft31425364---140 2.2kW-12V2802030405060---185 3kE-12V330192430374962--185 3kW-24V1606380------185 4kW-12V47014182330384860-185 4kW-24V23542506880100---185 4kW-12V48014182329374759-185 5kW-24V24042506880100---185 6kW-12V700-131621283550100185 6kW-24V370-3745607497--250 8kW-24V500-3241526684105-250 9.6kW-48V330-8098118154---300 10kW-24V650-192431394965130300 15kW-48V420-7492112148---25The ampacity issue is significant with shorter wireThis is European company, so their table ignores the relative rarity of 3AWG when they translated from metric 25mm2We can look at the example anywayThe apparent overload of the wiring is not necessarily out of line with the physical capacity of the wire, but it is a bit scaryIn the next slide we will look at the fuses vs. the 3 minute capacity of the wire from our time constant study. (This manufacturer recommends a maximum usage time of 3 minutes.)26Lets Zoom in and compare the recommended fuse and the E-11 wire rating:Length3 AWG Fuse VI-A105C WireRunning AmpsLoop Feet N-Eng Rm At 3 minutes Fuse/Wire Rate 3 min.270202001803051.11110.66280202001803051.11110.66330192501803051.38890.82160631301803050.72220.426470144001803052.22221.31235421301803050.72220.426480145001803052.77781.64240423251803051.80561.06527Compare Running Current to Recommended Fuse:Length3 AWGFuseAmp/FuseAmpsRatio270202001.35280202001.4330192501.32160631301.2308470144001.175235421301.8077480145000.96240423250.738528Clearly these thruster instructions are taking advantage of transient propertiesThere are some anomalies, but generally they are exceeding both the fuse and the wire ratings with apparent success.There is a backup protection in that these systems use an over-temperature sensor in the motor. It has a long time delay and senses the effect of overload or of excessive operating time. This allows the fuse to provide more short circuit protection while the thermal protection attends to overloads.The reliance on thermal overload devices is treated under NEC in section 430. Conductor sizes may be rated for less than the operating current if durations and demand factors are sufficiently low.29ABYC does not address these issues with transient loads..The topic is complexIt is not easy to address based on NEC or ABYCNEC is concerned with a wide range of motor sizes and AC voltagesThe few sections for DC motors do not give us something we can copy and paste into ABYC without further analysisThis manufacturers instructions very roughly follow the transient capacity for 3 minutes that we have been talking about but clearly violate it in many instances.

30What could ABYC standard changes look like that would address some of the issues here? Suggested wiring methods for motor loads designed for short term transient use:Motor shall be manually controlled with a momentary switch (deadman control).Battery switch or secondary disconnect shall be provided in case of motor control contact welding.Motor thermal overload protection shall be provided.Wiring shall at minimum have a 3 (5) minute capacity equal to motor running current.OCP shall be sized to not more than wire capacity at 3 (5) minutes.

31Thruster Table if Modified by Proposed RuleTT ModelCurrentAWG (Circuit loop Length in feet)Amps32100000000002x0000140 2.0KW-12V27020 ft31425364---140 2.2kW-12V2802030405060---185 3kE-12V330192430374962--185 3kW-24V1606380------185 4kW-12V47014182330384860-185 4kW-24V23542506880100---185 4kW-12V48014182329374759-185 5kW-24V24042506880100---185 6kW-12V700-131621283550100185 6kW-24V370-3745607497--250 8kW-24V500-3241526684105-250 9.6kW-48V330-8098118154---300 10kW-24V650-192431394965130300 15kW-48V420-7492112148---32Equipment Enclosure GroundingThere are a variety of reasons to ground or bond equipment related to transient faults

Panel and case grounding to prevent shock hazards (E-11)

Metal case bonding to prevent stray current corrosion elevated case potential. (E-11, E-2)

DC grounding of AC:DC devices to prevent excess current from DC faults in the AC grounding system. (A-31)33Bonding and Grounding11.16.1DC Grounding- If a DC grounding system is installed, the DC grounding conductor shall be used to connect metallic non-current-carrying parts of DC machinery and engine blocks to the engine negative terminal or its bus for the purpose of minimizing stray current corrosion and ensuring a fault current path in the event of a short circuit.

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DC Negative/ DC Grounding

35Another important class of transients involve faults and enclosure grounding in A-31This issue came up as a comment to A-31A-31 requires that metal enclosed chargers and inverters there is to be a DC grounding conductor as well as an AC grounding conductor.The significant purpose of the DC grounding conductor is to direct the current from a DC fault back to the DC system and not send the potentially high DC current into AC grounding wire.The DC grounding wire is to be sized at not smaller than one size less than the DC feed/charging conductors under present rules.Inverters are very sensitive to voltage drop, and need a supply that will support overloads during AC motor starting.This implies large supply conductors and therefore large grounding conductors. Sometimes impractically large grounding conductors.This leads to frequent non-compliance, making things less safe, not more safe.36DC Grounding of Chargers and Inverters A-31

37Adding the DC Ground Wire per A-31

38We were not able to do sufficient analysis to generate an amendment during this past years ABYC review.

What we know:Protection of the AC grounding conductor involves the size of the DC fuse and the current division between the AC grounding conductor and the DC grounding conductor.It does not depend on the size of the DC feeder conductor as the ABYC E-11 rule implies.The current division between the AC and DC grounding conductors depends on both the relative sizes of grounding wires, but also the lengths of the grounding paths back to the DC source negative.39NEC offers some Advice and AnalysisUnder Article 250 Grounding and Bonding there is a table 250.122Allows substantially smaller grounding wires than feeder wires in the larger sizes.Takes advantage of longer time constants and higher surge ratings in larger conductors.40NEC offers some guidance in 250.122NEC 250.122AmpacityperOCPCu. ConductorE-11 VI-A in Engine RoomAmpsAWG151430201238301051401051601051100868200610230041364003153500217960012088001/024210002/028141Wire size from 105C, Engine Room.41This allows much less than one size smaller!With small wire, a delay fuse may be slower to heat than the wire.With very large wire, the wire is much slower to respond than the fuse.The justification is also based on allowing a grounding conductor, during a fast clearing fault (5 seconds) to exceed the normal temperature rise by a factor of 2. (but NEC is built around 60C and 75C wire)They are dealing with the concern of overheating just the equivalent of the DC grounding conductor, not also considering the AC grounding conductor.42We can consider the AC Ground with a bit of math: AC output current < 0.065 of DC fuse ratingAssumption for AC conductors for an inverter:

DC current = 80% of DC fuse10:1 conversion ratiofor 12:120 V90% efficiency90% power factor43

44What could an ABYC standard in A-31 look like?DC Grounding conductor for Inverters and Chargers shall be sized in accordance with Table XX (Derived from NEC 250.122) based on the rating of the DC Over Current Protection Device.Exception: If AC grounding conductor length is less than the DC grounding conductor length, DC grounding conductor shall be increased by one size.45We could use NEC 250-122 in A-31We can use this in A-31 or something similar to it but maybe a bit more conservative.

This works for the inverters and chargers, but other considerations take precedence for windlasses and thrusters where stray current corrosion is the issue!

Need to separate the kinds of DC grounds for the problems they solve.46Do we need to examine E-11 for stray current corrosion?Based on ABYC E-2 Standard: A cathodic protection system shall be capable of inducing and maintaining a minimum negative shift of 200 mV relative to the corrosion potential of the least noble metal being protected.The target is to keep potential differences among metals under 200mV. That is about 1.5% voltage drop in a 12V system.That might imply even a larger grounding system to really be successful.Perhaps active potential monitoring would be more practicalA multi-input potential monitor with an alarm is a straightforward extension of existing meter technology.Requires a minimal sized sensing conductor from each device monitoredOnly effective when the boat is occupied, best practice to turn off devices that can be turned off when no one is aboard.47High AC ripple in DC systems is a repetitive transientWe see this typically in the DC feeders of inverters.Very little issue with modified sine inverters in marine.A bigger issue with sine wave inverters, so this is emerging as these are now much more popular.Can totally surprise you with some line frequency inverters as used in alternate energy.48High Ripple leads to an RMS heating greater than expected from the DC current reading.Shows up as overheated wireNuisance blowing of fusesEven failure of battery switches49An Example of the DC Current to an Inverter3kW Inverter, 26A AC outTrue RMS in DC is 380A, but DC AVG = 280A

50Most meters dont reveal RMS content of DCThis is about 135% increase in current over what would be measured.

That makes the heat in the wire, the fuse, the battery switch all increase to 180% of what you expect from DC measurement!

Some AC:DC clamp meters can measure RMS AC in the presence of DC.. The BSS 1800 can do it, but only if you measure DC first and then switch to AC and read that.

Add the squares of DC and AC components and take the square root to get the net effect.

51Yes! You can talk about Power Factor in DCThis inverter is supplying 26A at 118V = 3068WThe product of the DC RMS Amps times the DC Volts is 4560 VARsThis class of inverter is about 95% efficient = 3230 Watts input

The Power Factor = 3230/4560 = 71%52What about a Transient on top of a base load?The base load heats the wire or switch according to I2.The transient consists of the base current plus the additional current.If we have a base load of 100A on a battery switch rated at 350A, we will have a heat rise of only 8% of our rated rise!We can tolerate a 1 minute transient where the new current is SQRT of (1-0.08) = 96% of rating. A 650A rating would let us add 524A on top of the original 100A base current.53Some Handy Rules of ThumbEffective fault current from batteries is approximately 5X the CCA rating.Product of battery effective resistance and CCA is approximately 4.2000CCA battery => 2mOhm .Fuses blow and breakers trip about 125-150% of rating for US fuse ratings.Eds estimate for starting current = 1A/cu.in for gas, 2A/cu.in. for diesel (may be a bit high today with permanent magnet starter motors).

54Any Questions?Thank You!

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