CURRENT TRANSFORMER REQUIREMENTS FOR VA TECH REYROLLE RELAYS The internal burden of the c.t. (eg. Its secondary resistance) must be taken into account if a true equivalent overcurrent factor is to be established for a lower burden of the load.
1 Argus Argus 1 to 6 (Numeri (Numeric); c); 2TJM10 2TJM10 (Elect (Electroromechanical): I.D.M.T.L., Definite Time and Instantaneous Overcurrent and Earth Fault Protection A protection class c.t. must be used, eg in accordance with IEC 185 or BS3938. Typically the c.t. is specified by means of an accuracy factor and an overcurrent factor up to which the c.t. remains accurate with the maximum specified burden connected to its secondary, eg: 5P10
Accuracy Limit
:
Overcurrent Factor
Typically, c.t. requirements vary dependent on the project specific requirements. requirements. The following considerations considerations must be made. A)
C.T. C.T. Ratin Rating g – shoul should d be chosen chosen at lea least st e equa quall to the maximum continuous load current of the circuit. This includes any any emergency rating, rating, eg. of a po power wer transformer transformer where where typically one hour) one hour or two hour overload ratings are often provided.
B)
Accuracy Factor – Typically standard values of 5% or 10% are employed. 5% where current grading requirement are onerous eg. where the circuits being graded have similar ratings and there are several stages stages of grading. In these circumstances an accuracy of 5% assists in allowing small grading grading steps. Accuracy levels of 10% are acceptable where large grading steps can be tolerated and only a small number of grading steps are required.
C)
Overcu Overcurre rrent nt Fact Factor or – The factor factor should should be chosen to ensure:-
5VA
Maximum burden (at c.t. secondary level)
The accuracy limit is in percent, and although termed a composite error (which takes account of polarity error and magnitude error) is effectively the maximum maximum ratio error. The c.t. is more accurate at current current values up up to full load. load. Beyond full load, up up to the overcurrent overcurrent factor, the error will not exceed the specified value. The overcurrent factor is a multiple of the c.t. rating up to which the c.t. remains within the accuracy limit. Thus in the example example of a 5P10, 5P10, 5VA c.t., this will will transform primary current within the accuracy limit of 5% when a burden of 5VA (at rated current) is connected to the c.t. secondary, for a primary current up to 10x its rating. If the load burden is less than the rated burden a higher overcurrent factor can be tolerated, although not necessarily exactly in inverse proportion, i.e. half the burden, twice the overcurrent factor does not necessarily apply.
i
I.d.m. I.d.m.t.l t.l.. relay relay perfor performan mance ce is not undermined by the c.t’s inability to transform all the current into the secondary circuit.
ii
That any a.c. saturatio saturation n due due to a very very large large primary current is not so sever as to result in insufficient energy in the secondary waveform to prevent the relay from operating. Provided the above criterion are met it is not necessary to select an overcurrent factor which ensures the maximum fault current can be accurately transformed. transformed.
d) Burden Burden – this is the load load burden burden in VA, VA, at the rated rated c.t. secondary current, of all equipment connected to the c.t. secondary, including all pilot burden. Pilot burden can be significant, particularly for 5 amp rated rated c.t’s. c.t’s. Most modern modern protection protection relays (static or numeric) represent a fixed burden no matter what the setting. However, for electroelectromechanical relays, the burden may be dependent on the setting of the relay. Examples of typical applications are as follows. •
I.D.M.T.L. Overcurrent
a) For industrial industrial system systems s with relative relatively ly low fault fault current and no onerous grading requirements – a class 10P10 with rated burden to suit the load. b) For utility utility distribution distribution network networks s with relatively relatively high high fault current and several grading stages - a class 5P20, with rated burden to suit the load. Note: Where the maximum fault level is considerably considerably higher than the overcurrent factor and therefore the c.t. secondary current could be significantly lower than that equivalent to the primary current (due to a.c. saturation), it is necessary to consider any effect on the protection system performance, eg. grading margins. For i.d.m.t.l. applications, because the operating time at high fault current is approaching a definite minimum value, partial saturation of the c.t. at values beyond the overcurrent factor has only only a limited effect. effect. However, this must be taken into account in establishing the appropriate setting to ensure proper grading. •
Definite Time and Instantaneous Overcurrent
a) For industrial industrial systems systems with with requiremen requirements ts as for i.d.m.t.l. relays item (a) above, a class 10P 10. b) For utilites utilites as for for (b) above above – a class class 5P 10. Note: Overcurrent factors do not need to be high for definite time protection because once the setting is exceeded accuracy is not important. important. Often, however however there is also the need to consider instantaneous overcurrent protection as part of the same protection system and the settings would normally be of the order of 10x the c.t. c.t. rating or higher. higher. If they are higher higher than 10x then the overcurrent factor must be raised accordingly, eg. to P20. •
Earth Fault Protection
Considerations and requirements for earth fault protection are the same as for overcurrent. Usually the relay employs the same c.t’s eg. three phase mounted c.t’s connected in residual to establish the earth fault current.
The accuracy and overcurrent factors are therefore already fixed and for both these factors the earth fault protection requirements are normally less onerous than for overcurrent. A major consideration for an electro-mechanical relay is its burden at relay setting. Employing a low earth fault setting usually results in a high burden. burden. In these these circumstances the rated burden of the c.t. must be chosen on the basis of the requirements of the earth fault element. 2. Duobia Duobias s 4C21 (Electro-M (Electro-Mecha echanic nical): al): Transformer Differential Protection All CT’s should be of the low reactance type and the knee point voltage (Vk) should be equal to, or exceed twice the maximum steady state working voltage under any through fault condition. condition. To assess the steady steady state working voltage the impedance’s of the Duobias relays are ignored and only the c.t. winding and interconnecting lead resistance’s considered. As a general guide the knee point voltage Vk should equal or exceed:For Star connected CT’s - Vk equal or greater than 2I(A+C) For Delta connected CT’s -Vk equal or greater than 2I(B+3D) Where: I = Maxi Maximu mum m thr throu ough gh faul faultt c cur urre rent nt refe referre rred d to the the secondary winding of the star connected c.t’s with a three phase system fault. A = Second Secondary ary windin winding g resi resista stance nce of each each o off the the star star connected c.t’s. B = Seco Second ndar ary y wind windin ing g resi resist stan ance ce of of each each del delta ta connected c.t. C = Resi Resist stan ance ce of of eac each h le lead ad betw betwee een n the the star star connected c.t. terminals and the relay terminals. D = Resi Resist stan ance ce of of each each lea lead d betw betwee een n the the delt delta a connected terminals and the relay terminals. Class ‘X’ current transformers to BS 3938 (or class TPS to IEC 44-6) can be specified to meet the above requirements and this type are recommended. 3. B3/5B3 (Electro-mechani (Electro-mechanical);DAD cal);DAD (Static): High Impedance Differential and Restricted Earth Fault Circulating Current Protection The basic requirements are: 1 .All the the current current transforme transformers rs should should have have ident identical ical turns ratios.
2 The Kne Knee e point point volta voltage ge of the the curre current nt transformers should be at least twice the relay setting voltage. voltage. The knee point voltage voltage is expressed as the voltage at fundamental frequency applied to the secondary circuit of the current transformer which when increased in magnitude by 10% causes the magnetising current to increase by 50%. 3 The current current transf transformers ormers should should be be of the low leakage reactance type. Generally most modern current transformers are of this type and there should be no difficulty in meeting this requirement. Low leakage reactance current transformers have a jointless core with the secondary winding evenly distributed along the whole length of the magnetic circuit, and the primary conductor passes through the centre of the core. Class ‘X’ current transformers to BS 3938 (or class TPS to IEC 44-6) can be specified to meet the above requirements and this type are recommended. 4. Duobia Duobias s M (Numeric) (Numeric):: Transform Transformer er Differential and Restricted Earth Fault For high speed operation under all fault conditions the minimum current transformer knee point voltage should equal or exceed: Vk = 4I(A+C). Where: I = Eith Either er the the maxi maximu mum m thr three ee phas phase e thro throug ugh h faul faultt current (as limited by the transformer impedance) or the high-set setting, whichever is greater A = The The sec secon onda dary ry win windi ding ng resi resist stan ance ce of each each star connected c.t. C = The The c.t. c.t. sec secon onda dary ry loo loop p lead lead res resis ista tanc nce e for for internal earth faults For restricted earth fault protection it is recommended that all c.t’s should have an equal number of secondary turns. Line c.t’s c.t’s are normally normally star connected and standard ratios can be selected according to the transformer rating, ratios need not be exact provided they are within the range of the Duobias-M relay current setting ranges and do not cause the c.t. or relay thermal ratings to be exceeded. Ideally the line c.t. ratios should be selected to allow Duobias-M relay settings for c.t. ratio correction factors to be employed in order to balance the secondary current to normal relay current. This allows maximum sensitivity to be achieved for internal faults. Class ‘X’ current transformers to BS 3938 (or class TPS to IEC 44-6) can be specified to meet the above requirements and this type are recommended.
5.
Solk Solkor or R/Rf R/Rf (Elect (Electroro-me mecha chani nical cal): ): Pilo Pilott Wire Wire,, Feeder Current Differential Protection
The minimum knee point voltage of the line current transformers is given by: Vk = 50 + If (Rct + 2Rl) In N Where In = Rated c cu urre rrent of of rre elay, am amps If = Prim Primar ary y curr curren entt unde underr maxi maximu mum m stea steady dy stat state e through fault conditions N = Current transformer ratio Rct = Second Secondary ary resist resistan ance ce of the curren currentt tran transfo sforme rmerr in ohms Rl = Cabl Cable/ e/wi wiri ring ng resi resist stan ance ce betw betwee een n the the curre current nt transformers and the relay summation transformer, for each single wire (route length), in ohms Generally it is not recommended that any other equipment burdens should be included in the current transformer circuit in order to avoid any possible maloperation due to to through through faults. faults. However, in some some instances the protection design often requires the inclusion of starting relays for the Solkor protection and occasionally the addition of i.d.m.t.l. protection to the same c.t’s for backup protection. protection. In such cases the extra extra burden should be carefully established and included in the calculation. calculation. The additional additional burden on each phase should be reasonably balanced. The secondary magnetising currents of the current transformer at opposite ends of the feeder should not differ by more than In/20 amperes for output voltage up to 50/In volts. To ensure good balance of the protection the current transformers at the two ends should have equal ratios. Close balance of ratio is provided by current transformers to IEC44-6, Class TPS (BS3938, Class X), where ratio error is limited to ±0.25% and these are recommended. 6
Solk Solkoror-M M (Num (Numeri eric): c): Feeder Feeder Curre Current nt Diffe Differen rentia tiall Protection
The current transformers at the two ends should have similar design parameters and performance characteristics. In addition addition the secondary secondary burden of the two current transformers transformers should be kept similar. similar. This will then allow a low value of stability factor to be used, hence reducing the knee point voltage requirements of the current transformers. The minimum knee point voltage for the line current transformer is given by: Vk = k x X/R x If/N x (Rct + 2Rl + Rb)
where: k = stability X/R X/R = the the X/R X/R rat ratio io for for the the maxi maximu mum m thro throug ugh h fault conditions. The value of this transient factor depends upon the sum of the source and transmission circuit impedance’s. If = prim rimary curre rrent under ma max ximum through fault conditions (amps) N = current tr transformer ra ratio Rct Rct = seco second ndar ary y res resis ista tanc nce e of of the the curre current nt transformer (ohms) Re = lead lead resi resist stan ance ce b bet etwe ween en the the cur curre rent nt transformers and the relay ohms) Rb = burd burden en of rela relay y (oh (ohms ms)T )The he ac burd burden en of the relay per phase is 0.05V at 1A for 1A tap = 0.05 ohm 0.3VA at 5A for %a tap = 0.012 ohm It is not recommended that any other burden should be included in the current transformer circuit, but where this cannot cannot be avoided the additional burden burden should be added to those listed when determining the current transformer output voltage required.
Rl Rb
It is not recommended that any other burdens should be included in the current transformer circuit, but where this cannot be avoided the additional burden should be added to those listed when determining the current transformer output output voltage required. required. In addition to the above, the secondary magnetising currents of the current transformers at opposite ends of the feeder should not differ by more than In/20 amperes for output voltages up to 50/In volts. To ensure good balance of the protection the current transformers at the two ends should have equal ratios. Close balance of ratios is provided by current transformers to IEC44-6, Class TPS, ratio error limited to ±0.25%, and these are recommended r ecommended.. •
Relay AC Burden per phase 0.05VA at 1A for 1A tap = 0.05 ohm 0.3VA at 5A for %a tap = 0.012 ohm
•
Stability Under through fault conditions the relay will be stable with fault current equivalent to 50 times the normal current rating in use.
8
THR THR (Sta (Static tic): ): Dista Distanc nce e (Im (Impe pedan dance) ce) Protec Protecti tion on for Transmission Circuits
In addition to the above, the secondary magnetising currents of the current transformers at opposite ends of the feeder should not differ by more than In/20A for output voltages up to 50/InV. For example, consider a 33kV feeder with a worst case through fault of 8kA with a X/R of 10. The minimum current transformer knee point required, given a turns ratio of 1/400, secondary CT resistance of 2Ω, and lead burden of 1 Ω, is :-
= Lead Lead resi resist stan ance ce betw betwee een n the the curr curren entt transformers and and the relay (ohms) = Burde rden of relay (oh (ohms)
For high speed operation and accurate impedance measurement the c.t’s should be Class TPS to IEC44-6 (Class X to BS3938) and have a knee point voltage (Vk) equal to or greater than the following:-
Vk ≥ 0.3 x 10 x 8000/400 x (2 + 2x1 + 0.03) Vk ≥ 242volts 7
Micr Microp opha hase se-F -FM M (Num (Numer eric ic): ): Curr Curren entt Differential Telephase Protection
The minimum knee point voltage for the line current transformers is given by: Vk = k x X x If x (Rct + 2Rl + Rb) R N Where: k = Stability factor = 0.8 X/R X/R = The The X/R X/R rati ratio o for for the the max maxim imum um thr throu ough gh fault condition. The value of this transient factor depends upon the sum of the source and transmission circuit impedance’s. If = Prim Prima ary curr curre ent unde underr ma maxi ximu mum m ste steady ady state through fault conditions (amps). N = Current transformer ratio Rct Rct = Seco Second ndar ary y res resis ista tanc nce e of of the the curre current nt transformer (ohms)
Vk ≥ If [R1 + R2 + X (R3 + R2)] R Where: If = Seco Second ndar ary y fau fault lt curr curren entt for for faul faultt at at end end of Zone Zone 1 R1 = Resi Resist stiv ive e bur burde den n of of THR THR rela relay y (Se (See e tab table le below) R2 = Resi Resist stan ance ce of of conn connec ecti ting ng lloa oads ds plu plus s resi resist stan ance ce of the C.T. secondary winding X = Rati Ratio o of of rrea eact ctan ance ce to resi resist stan ance ce of the the sys syste tem m for R a fault at the end of Zone 1 R3 = Cons Consta tant nt dep depen endi ding ng o on n impe impeda danc nce e set setti ting ng o off Zone 1 To calculate values of R1 and R3 for 2A or 5A relays, divide values in the following table by 4 and 25 respectively.
Relay nominal rating (A)
Setting of Zone 1
R1
1
0.8 to 4
0.3
0.3
0.2
4 to 8
0.4
0.4
0.3
8 to 16
0.8
0.6
0.5
16 to 24
1.5
0.9
0.5
24 to 48
3.8
1.2
0.9
9
R3 Earth
Phase
Fault
Fault
Ohme Ohmega ga (Nu (Nume meri ric) c):: Dist Distan ance ce (Im (Impe peda danc nce) e) Protection for Transmission Circuits
For high speed operation and accurate impedance measurement the c.t’s should be Class TPS to IEC446 (Class X to BS3928) and have a knee point voltage (Vk) equal or greater than the higher of the following two expressions: 1) Vk ≥ K • Ip (1+ Xp) (0.03 + Rct + Rl) N Rp For phase-phase faults 2) Vk ≥ K • Ie (1+ Xe) (0.06 + Rct + Rl) N Re For phase-earth faults Where: Ip le N Xp/R Xp/Rp p
the Xe/Re Xe/Re Rct RI K
=Phase fault current calculated for Xp/Rp ratio at the end of zone 1 =earth fa fault cu current ca calculated fo for Xe Xe/ Re Re ratio at the end of zone 1 =C.T. ratio =pow =power er sys syste tem m react reactan ance ce tto o resis resista tanc nce e ratio ratio for the total plant including the feeder line parameters calculated for a phase fault at end of zone 1 =simil =similar ar ratio ratio to abo above ve but but calc calcula ulated ted for an earth fault at the end of zone 1 =C.T. internal resistance =lead bu burden, C. C.T. to to Oh Ohmega te terminals =Factor chosen to ensure adequate operating speed and is <1. K is usually 0.5 for distribution systems, a higher value is chosen for primary transmission systems. Reyrolle Protection should be consulted.
Both Vk values should be calculated and the higher value chosen for the c.t. to be used. 10
GAMMA GAMMA (Nume (Numeri ric): c): Genera Generator tor Prote Protecti ction on
a)
Two off 3 phase phase Inputs Inputs (Line (Line end and Neu Neutra trall End):
The current transformer minimum requirements depend on the protection application, the functions employed and the primary circuit configuration.
For satisfactory operation of all functions except the low impedance biased differential function, the use of a class 5P20 to IEC60185, or any equivalent, would be satisfactory for any application since the fault levels never exceed 20 x the the c.t. rating. rating. The VA rating is chosen to allow for all the circuit burden (eg. c.t. secondary cabling and relay burden). For stability of the low impedance biased differential function it may be necessary to provide a design which ensures neither of the two 3-phase sets of c.t’s are overfluxed in the event of re-occurring high magnitude faults with high X/R ratio source source impedance. In these circumstances, where high levels of d.c. component current, long time constants and a long operating time for the network protection may occur, the c.t’s can be left with a high level level of remnant flux. Any subsequent subsequent faults may then cause one of the c.t’s to fully saturate and the differential function mal-operate. If this is possible, e.g. for a directly connected generator (no generator transformer), where the two sets of c.t’s may be supplied by different manufacturers, where there is a multi-shot delayed auto-reclose scheme on feeders local to the grid connection, and the differential setting chosen is very sensitive, it is recommended that any low reactance c.t’s (ie with high remanance factor) factor) should have knee point voltages compliant with the following formula:Vk = 50ln (RCT + 2RL + RR) where maximum through fault current = 10 x ln with maximum X/R = 120. Minimum Vk to be 60 Volts. Vk = 30ln (RCT + 2RL + RR) where maximum through fault current = 10 x ln with maximum X/R = 60. Minimum Vk to be 60 Volts. Where: Vk = Knee point voltage ln = Rated current X/R = The X/R ratio ratio for for the the maxi maximum mum throug through h fault fault condition. RCT = Second Secondary ary resist resistan ance ce of of the the curren currentt transformer (ohms) RL = Lead Lead res resis ista tanc nce e be betw twee een n th the e curr curren entt transformers and the relay (ohms) RR = Resist Resistan ance ce of any any other other protec protectio tion n functi functions ons sharing the current transformer (ohms) Where all the onerous conditions described above are not required to be met and the c.t. accommodation facility is limited, the requirements can be reduced, in these circumstances contact VA Tech Reyrolle Protection for advice. b)
Neut Neutra rall Ear Earth th C.T. C.T. Inpu Inputs ts:: For solid earthed (eg. direct connected generator), use the same as (a) above
For impedance earthed neutral, a lower specification can be employed eg 5P5. Use of C.T’s Common to More Than One Relay Generally the c.t’s employed for generator protection should be dedicated to that one duty, for security of the protection. Technically however there is no reason why other equipment may not share the same c.t’s, except that the additional burden should be taken into account and also that the c.t’s for the three phase inputs should have a reasonably balanced burden on each phase. This ensures no possibility possibility of mal-operation mal-operation of the differential function. For the requirement of redundancy, there is no problem with the performance of either relay when connecting two Gamma relays in series. However, we recommend that the redundancy philosophy be extended to include the c.t’s, ie. use separate c.t. secondaries. Use of duplicate Gamma relays, particularly on high rated generator units (eg over 15 MW) provides a high level of security and integrity which is still cost effective. 11
RHO RH O (Nume (Numeric ric): ): Moto Motorr Prote Protecti ction on and and Electrical Plant Thermal Overload
11.1 RHO for low low voltage voltage 3 phase A.C. A.C. motors The RHO 0 relay is compatible with CT’s having either 25mA or 5A secondaries. 25mA rated output c.t’s are recommended for motor currents up to 100A and 5A up to 3000A For 5 amp relay input rating some motor control systems result in high multiples of rated current flowing during the start up period, and for some motors the run-up time may be very long (eg 60 seconds). Because of this there are some limitations on the motor rating that can be utilised with a particular c.t. rating due to the thermal capacity of the relay c.t. input input terminals. If the motor rating is limited to less than 75% of the c.t. rating there is unlikely to be any problem with overheating no matter how onerous the starting current, starting time or number of starts in a period of time for practical considerations. The c.t. classification should be 10P10 or better (eg. 5P10 for improved accuracy) and have a rating to suit the c.t. secondary secondary total burdens, burdens, ie. C.t. C.t. leads, RHO 0 relay and any equipment connected in series with the relay.
The RHO c.t. input circuit burden is fixed and is no greater than 0.25VA, eg 0.01 ohm at 5 amp and the rated burden is therefore established by selecting a value in excess of the c.t. secondary circuit loading eg. for phase inputs:Rated VA
≥
(In)² (2Rl + Rb + R1)
Where: In = Rate Rated d seco second ndar ary y curr curren entt Rl = Second Secondary ary lead lead burde burdens ns per pha phase se (ohm) (ohm) Rb = Rela Relay y cir circu cuit it bur burde den n ≤ 0.015 ohm for 5 amp ≤ 11 ohm for 25 Ma R1 = Other Other equip equipmen mentt burde burden n (ohm) (ohm) per per phase phase For earth fault detection RHO 0 has a separate input which can be employed either from a residual connection of the phase inputs or a separate core balance c.t. (the preferred option). option). The relay is set set to the selected selected mode eg. Residual Connection or CBCT. In residual connection the trip setting and primary c.t. rating setting establish the pick-up level. In CBCT mode the setting range is 30mA to 3000mA, and the CBCT ratio is chosen with this in mind to establish a required required primary trip current. current. The actual actual current input range at the relay terminals is 0.06 to 6.0 MA. The class and rating are selected as per the phase input c.t’s. The rated burden burden is establish from:from:VA ≤ (In)² (2Rl + R1 ) [References as before] For the earth fault c.t. input Rb
≤
300 ohm
If the residual connection mode is employed the e.f. c.t. input burden should be added to the phase c.t. burden since this is a significant significant value. value. For this form of connection a stabilising resistor may be required in the earth fault c.t. input, e.g. if the setting is instantaneous. This resistance must be included in the value for R1. However residual connection is not recommended. 11.2 11.2
RHO 3 for high voltage voltage 3 phase phase A. A.C. C. moto motors rs and electrical plant
Thermal Overload The c.t. class recommended recommended is 5P10. 5P10. This provides provides accurate measurement (maximum error of 5%) for overloads and also for high current magnitudes beyond typical motor stall current (eg. 6 x full load current). The rated burden is established by selecting a value in excess of the c.t. secondary circuit loading, eg:Rated VA
≥
(In)² (2Rl+ Rb + R1)
Where: In = Rl = Rb = R1
=
Rated secondary current Secondary lead burdens per phase Relay circuit burden (See table below) Other equipment burden per phase
RHO 3 C.T. Input Burdens
5A Phase 1A Phase 5A Earth 1A Earth
AC Burden
Impedance
£ £ £ £
£ £ £ £
0.2VA 0.05VA 0.4VA 0.2VA
0.01 W 0.05 W 0.02 W 0.2 W
For earth fault detection RHO3 has a separate input which can be employed either from a residual connection of the phase inputs or a separate core balance c.t. (the preferred option). There is no selection made within the relay, the primary current setting is a function only of the relay current setting and the c.t. ratio. If a residual connection is employed and the earth fault setting chosen is both sensitive (e.g. less than 0.5In) and instantaneous, it is recommended that a stabilising resistor be employed in the earth fault input circuit. This must then be taken taken into account in establishing R1. For residual connection arrangement, Rb will be a summation of the phase fault and earth fault input burdens. 12
PHI PHI (Nu (Nume meric ric): ): Point Point-on -on-W -Wave ave Circu Circuit it Breaker Controller
The current transformer inputs do not need to be employed for the point-on-wave function and can be left unconnected. unconnected. If connected connected to the c.t’s c.t’s the PHI unit enables the current profile to be monitored for continuous load load and switching switching conditions. The class of c.t. is not important and either instrument or protection class c.t’s can be employed, eg any c.t’s already employed employed for for another another function. Dedicated ct’s are not n necessary. ecessary. It is only necessary necessary to ensure the additional small burden of the PHI relay is included in the requirements of the c.t’s being employed.
