Ask the Expert

Find your answer in the categories below or simply type in your question here and one of our specialized engineers will be happy to reply.

For your convenience, we have created PDF booklets with the most frequent questions and answers in each category.

Arc Flash Hazard and High Resistance Grounding

Sir, I am planning to buy a 3 phase ups with isolation transformer for our critical machines. We understand that the star point of output of isolation transformer to be grounded which ofcourse will act has neutral for my loads. My worry is if for any reason the ground becomes week (soil conditions is really bad)or goes missing what will happen to the connected loads and how to protect it. I can't use the bypass system in ups for neutral cause i have two sources for power which is again isolated by four pole breakers.

Adding a grounding conductor from the transformer ground to connect all the equipment enclosures will remove the uncertainty associated with relying on the Earth return (through soil) in the event of a fault. Neutral will carry the unbalanced load current back to transformer and the grounding conductor will carry fault current back to the transformer. This will be a lower impedance path than the earth and will assure that equipment is safe to touch as well. This is normally done using a green colored cable. For detailed explanation refer to IEEE Std 1100 (Emerald book).

Why HV system is always solidly earthed ,MV system is resistance earthed and again LV system is solidly earthed.

Solid earthing lowers the power conductor insulation requirements. Insulation level only needs to be suitable for the phase to ground voltage and not line to line voltage. This has major cost benefit for insulators on transmission lines, cables and lightening arrestor ratings. Solid earthing reduces the impedance in the return path in the event of a phase to ground fault. So lot of fault current flows which can then operate protective relays. With solid earthing no transient overvoltages due to earth faults occur. So solid earthing is preferred for the HV systems However the consequence of high ground fault current with solid grounding is the very high energy dissipation at the fault point, which can be devastating. For MV systems, insulation system suitable for phase to phase voltage can usually be applied. The penalty for higher insulation level at MV is not significant and benefit of low fault damage far out weighs the cost. For systems up to 5 KV in many jurisdictions continuous operation of the faulted circuit is permitted if the current in the ground path can be 10 A or less. This provides a major benefit to the power systems where trip out due to a phase to ground fault cannot be tolerated and causes consequential loss due to power interruption. LV systems have been solidly earthed to maximize the phase to ground fault current so that circuit protective devices such as fuses and breakers will open. With modern electronic relays detecting, and locating ground faults can be done even when ground currents are small. Hence in the Industrial LV distribution systems where neutral conductor is often not required, the practice of limiting the earth fault current to less than 10 A, allows power continuity. The fault can be detected, alarmed and located using electronic relays. This allows faulted feeder to be serviced when convenient. So LV 3 phase 3 wire systems can be high resistance grounded and such systems are permitted by the installation codes In most 3 phase 4 wire LV systems where neutral conductor is distributed Solid earthing of Neutral is mandated. Since with a phase to ground fault the neutral will elevate in potential when LV system is resistance grounded. So resistance grounding of LV 3 phase, 4 wire circuits is not used and often not permitted by codes.

Are there disadvantages to utilizing a high resistance grouding scheme on 4000A 1000V DC switchgear? Does this class of DC gear have arc-flash hazards the same as AC switchgear?

No, I do not see any disadvantages in high resistance grounding 1000Vdc switchgear. Where would you apply the High Resistance grounding? Is center tap available to allow /- 500V? This would be the most convenient place to apply HRG and you can monitor the system for ground faults.

Grounding of Standby and Emergency Power Systems

In big combined cycle power plant, is it recommended to ground the UPS neutral or it is the best practice to keep the neutral floated so that incase of ground fault the reliability of the UPS is maintained.As I understand, incase the UPS panel board is having single pole breakers ungrounded neutral will cause violation of the NEC requirement to have an overcurrent device in series with each ungrounded conductor.Please give your valuable feedback.

Yes, Single phase, Line to Neutral loads fed by single pole breakers are not permitted for ungrounded systems in the NEC. To meet the code the UPS output will need to be solidly grounded. If there are no single phase or 4 wire loads to be fed, then the UPS could be feeding only 3 pole breakers and UPS output can be kept floating or high resistance grounded. To meet NEC, Ground fault indicators needs to be installed.

Low and Medium Voltage Distribution Systems

Transient over-voltages on ungrounded electrical systems due to intermittent ground fault exist. The high resistance grounding reduces the potential overvoltage significantly. But what about surge suppressors? Do they protect a medium voltage system from the overvoltage? Do they protect the system with a high resistance grounding when the grounding resistance is not selected correctly and a capacitive charging current is several times higher than the resistor current?

The transient over-voltage caused by intermittent ground fault on ungrounded system is due to the distributed capacitances to ground on the three phases. The magnitude can be 5 to 6 times phase to phase voltage. The Neutral Grounding Resistor allows this charge to be dissipated and holds the neutral above ground at phase to neutral voltage thus allowing the voltage to ground on the unfaulted phases to increase from phase to neutral value to phase to phase value and this will exist continuously as long as the fault is present. So there is no over-voltage beyond the phase to phase voltage magnitude. However very short duration transients can still occur and exist on the three phases such as those caused by the lightning strikes and switching. Surge suppressors are normally intended to clip and absorb very short duration (millisecond to microseconds) transient over-voltages such as those caused by lightning strike and switching. They cannot handle continuous excessive voltage. Their clipping voltage is also quite high. It could be more than 1.5 X the normal steady state peak voltage. So when TVSSs are applied on HRG systems they need to be rated for line to line voltage. They will then clip transients above their continuous rating.

I am sizing an NGR for a 132kV/11kV,Dyn11 40 MVA transformer. The biggest motor connected in the 11 kV system is 5.3MW slip-ring motor & Smallest one is 300 kW. I am looking for a low resistance grounding system. What should be the current limiting value for the NGR. How it is arrived. What are the criteria?

I recommend using Low-Resistance Grounding (LRG) due to the voltage and capacity. At 11kV and 40MVA, the capacity (MVA) indicates the system is large (in terms of total length of all of the feeder cables, which are typically the greatest contributor to system capacitive charging current) and the voltage induces higher amounts of system capacitive charging current. The next largest contributor to system capacitive charging current is surge arrestors/capacitors on generators. This can be estimated and measured, see our Application Guides, particularly "Ground Fault Protection on Ungrounded and High-Resistance Grounded Systems" for additional information on our website I would recommend 200A as this is becoming the industry standard. The NGR is determined by taking the line-to-neutral voltage (11kV / 1.73 = 6.36kV) divided by desired current (200A) to get ohms (31.8ohms). Most people only allow the fault to be on the system for 10 seconds or less. So, the NGR would be rated for 200A / 10sec. Just make sure that the protective relaying scheme clears the fault within 10 seconds. In general lower the fault current lower is the damage at the fault. Therefore it is desirable to keep the fault current as low as possible. If MV motors are being protected then keeping the fault current low also helps in lowering the damage to the laminations at the fault point in the event of a fault in the stator winding. In wye connected motor windings the driving voltage for the ground fault reduces as the fault location moves closer to the star point hence ground fault relay must be set sensitive enough to detect the fault and sufficient amount of current must flow. This will also dictate how low you can go with the resistor let through current.

Neutral Grounding Resistors - Design and Application

I had calculated the resistance of NGR 10Ohm for 10 sec for 400A current for 6.6kV system. Now I need to calculate the mass of resistance required considering Stainless steel ANSI 304 Grade? How to calculate that?

Each company calculates this differently, and is usually proprietary information. One thing to consider using 304 material, the resistivity increases due to heat rise. This means that the ground fault current will start out at 400A and then drop to ~250A after 10 seconds. Hopefully the decrease in current doesn't leave the pickup range of the relay. A change in current of this magnitude falls outside the recent recommendation issued by CSA in TIL.No.34.

What do you mean by capacitive charging current? Is it the max generator current, to create the magnetic field for your generator?

No. Capacitive charging current is current created by the system, almost solely from feeders and surge arrestors/capacitors. Think of a feeder, which is in this case is a cable in a conduit. If you physically look at the installation, the two (cable and conduit) are parallel to each other AND at different potentials, cable = phase voltage and conduit = ground potential. Both are separated by an insulating material. The result is a very big capacitor, at least in terms of physical size. When you energize the system, current flows from the phase conductor to conduit (actually, this isn't technically true, it actually causes a displacement of charge within the dielectric say the academics, in the real world, it is called current) through the dielectric, you just can't see it or measure it because it is happening ALL along the cable. It doesn't just flow in one spot, it is equally distributed along the cable. So when you have an Ungrounded system and a ground fault occurs, the phase voltages with respect to ground changes on the 2 unfaulted phases, say from 277V to 480V. What happens to the charge within the feeder capacitance? It rises from 277V to 480V, we are now CHARGING the capacitance. In this case, we now can measure it because there is a single spot (the ground fault wire). So, the easiest way to measure a system capacitive charging current is to Unground the system, place a ground fault on the system THRU a fast-acting fuse (~10A or less) and measure the current in the wire. This is your system capacitive charging current since it returns to the other 2 phases thru the insulating material, or dielectric. Just remember, this is 3*Ico, if you want just Ico, you must divide by 3.

System Grounding

Dear Mr Expert, We are doing a project in Mexico in which LV system is 440V 3 phase 3 wire that is IT earthed system. We have to supply some LV switchboards, MCCs and PDBs. What kind of earth fault protection system has to be provided? As far as I understand in IT system for the first ground fault only alarm and in the second (healthy phase) ground fault trip has to be provided. Can u please tell me the product to be used for this?

IT systems (or Ungrounded Systems) have advantages and disadvantages that have been very well documented for several decades. The advantages are continuous operation (no downtime due to ground faults) and no arc flash hazards during ground faults. However, it impossible to effectively locate a ground fault and large transient over-voltages may occur during intermittent (or arcing) ground faults or resonance conditions. So, many industries have retrofitted existing plants / facilities from Ungrounded to High-Resistance Grounded systems, which provide a method to effective locate ground faults and prevent transient over-voltages. In fact, one major manufacturing company with over 50 plants worldwide using HRG has a policy that each fault must be located in 1 hour. To answer your question, I would recommend a HRG system. HRG systems have all of the advantages of an IT system, however, it does not have any of the disadvantages. A HRG system does exactly what you are looking for only better. It will alarm on the first ground fault, and ONLY trip (1) feeder if a second ground fault occurs by shunt tripping the lesser priority feeder. (In an IT system, both feeders will trip since it is essentially a phase-phase fault.)

As you know in unbalanced 3 phase systems there would be a residual current which would pass through the neutral of the system. In such systems how can the grounding resistor affect the normal operation of the system? Is there any power delivery decrement to the load due to the limitation of the neutral current? Does the replacement of the system voltage neutral point affect the performance of the system?

In power systems with a resistor connected between neutral and ground, or resistance grounded, there cannot be any line-to-neutral loads. The neutral is not allowed to be distributed. So, there is no return path. Only 3phase and/or 2phase loads are permitted.

would like to know answers from you as questions below: How do youacan select zig-zag transformer grounding or wye broken delta for Delta system grounding ? How do you calculate kVA rating & voltage ratio of the zig zag transformer and wye/broken delta ? (If voltage system rating of 6.6kV and we will select NGR rating of 125A) Which type of the zig zag transformer and wye/broken delta do you recommend?

A zig-zag creates a neutral point. Advantage is that it is physically and electrically smaller than the wye-broken delta, so should be less expensive. Disadvantage is that there are only a couple of manufacturers and UL/CSA is not always available. Also, zig-zags only create neutral, so for a 4160V system, the neutral point would be 2400V. You could not add a 59 relay to this resistor or PULSE to locate ground faults. With a wye-broken delta, the secondary can be any voltage you choose, so the resistor will be < 240V, so you can use a relay and/or low-voltage CT and you can PULSE the low voltage resistor to locate ground faults. In your 6.6kV system and 125A, pulsing is not recommended. So, I would suggest a zig-zag transformer with a rating of 125A * The line-to-neutral voltage (6.6kV / 1.73 = 3815V) = 476.9kVA. Since it is 125A, I am assuming that it is rated for only 10seconds, so you can de-rate transformer by a factor of 10. So, new rating is 476.9kVA / 10 = 50kVA. Depends on your environment, but oil-type typically has much longer lifespan that dry type and are better for harsh environment. However, they are more expensive and require some maintainence.

Where solid grounding is more favoured?

The only reason to solid ground is for line-to-neutral loads.

How can we detect earth fault on floating earthing system through Core balance ct

Any energized conductor will have capacitance to ground so in three phase system there are three balanced distributed capacitances to ground due to natural conductor insulation. In ungrounded system these three capacitive charging currents, Ico, are normally balanced and there is no net ground current flow. In an earth fault as the voltage to ground on the faulted phase goes to 0 the charging current in that phase also goes to 0 where as the charging current increases in the other two phases which are driven at line to line voltage magnitude. And they add up to become 3 Ico. This charging current can be detected by a Zero Sequence Sensor ( also called Core balance CT ) to indicate earth fault. These currents are normally 0.1 A to 2 A in LV systems and up to 20A in MV systems.

I would like to know, what voltage you would read if you placed you leads from L1,L2, or L3 to ground of the 460 volt AC, 3 phase power system. Y connected ?

If the Y connected system is solidly grounded then you will read 266 V from line to ground. If the Y connected system is ungrounded or high resistance grounded and the system does not have a ground fault then you also read 266 V In the event that there a fault on one phase then the faulted phase will show low voltage near zero and the other two phases will read near 460 V

How can we measure earth fault and unbalance current in Ungrounded system.And how does open delta PTand core balance ct functions against faults.what effect on open delta PT.

In ungrounded system the phase to ground voltages of the three phases change when there is a earth fault. Under normal condition Phase to ground voltage = Phase to phase voltage / 1.73 and it is the same for the three phases On the occurrence of an earth fault The phase to ground voltage on the faulted phase is zero and for the other two phases it becomes equal to phase to phase voltage. The unbalance in Phase to ground voltages is measured by YY connected PTs and is used to indicate earth fault. Open delta connected Pts are connected Line to Line and do not see this change and are unaffected hence cannot be used for Earth Fault detection. Any energized conductor will have capacitance to ground so in three phase system there are three balanced distributed capacitances to ground due to natural conductor insulation. In ungrounded system these three capacitive charging currents, Ico, are normally balanced and there is no net ground current flow. In an earth fault as the voltage to ground on the faulted phase goes to 0 the charging current in that phase also goes to 0 where as the charging current increases in the other two phases which are driven at line to line voltage magnitude. And they add up to become 3 Ico. This charging current can be detected by a Zero Sequence Sensor ( also called CB-CT ) to indicate earth fault. These currents are normally 0.1 A to 2 A in LV systems and up to 20A in MV systems

What is the impact, if any, on moving equipment designed for a plant with a floating ground or ungrounded secondary to a plant that has a true grounded system. My thoughts are, it shouldn't really matter, but I could be mistaken.

In your case (from an Ungrounded system to a solidly grounded system) NO, it does not matter. However, if you were going the other way (from SG to a UNG system), then Yes it would matter. During normal operation, it more than likely will not matter, however, during a ground fault it will. In an Ungrounded system, the faulted phase voltage collapses to ground potential (or ~0V) and the unfaulted phases rise to phase-to-phase voltage with respect to ground. For example, a 480V system will have ~277V phase-to-ground voltage during normal operation, so it should work OK. However, a ground fault on A-phase makes its voltage go to 0V and the other two phase will rise from 277V to 480V phase-to-ground. Since this doesn't happen on a solidly grounded system, anything rated only 300V phase-to-ground will explode, such as TVSSs, VFDs, meters, etc.

Is there any danger in running a 480 volt ungrounded system in an old manufacturing plant? Should we ground the system?

The main danger in running a 480 V. ungrounded system is that when a ground fault occurs the only indication you will have is the 3 lights. The voltage on the ungrounded phases will increase to 480 V. with respect to ground, the voltage on the grounded conductor will be 0 V. with respect to ground. With this system the only way to indicate the presence of a ground fault will be when 2 lights are of greater brilliance than the faulted phase light. In order to locate the ground fault you must cycle every feeder breaker until all three lights appear at equal brilliance again. Once this is done you continue down that feeder until you find the fault. This sounds very easy to do, but proves to be very difficult in the real world. The plant is normally ungrounded because it is a continuous operational plant and isolation due to a ground fault should be avoided. This unfortunately translates to location of a ground fault. The only way to locate the ground fault is through cycling of the feeder breakers. This is also what you are trying to avoid. So at the end of the day the ground fault remains on the system, because there is no easy way to locate it. This is dangerous because any maintenance being performed on the system in a grounded state is subject to full line to line potential with respect to ground. The good news is that there is a solution. Ungrounded Facilities can be easily converted to High Resistance Grounded Facilities and the detection and location of a ground fault can accomplished without power interruption.