Fuse Selection Surge Applications

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© 2006 E aton C orporation

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WHITE PAPERProportioning Surge Protection and Fusing

- Fuse selection for surge applications

TVSS Engineering Group,Eaton Power Quality,

2 Kent Rd, Mascot,NSW Australia

Document: Fuse_selection_surge_applications.pdf

Prepared by: Jon Point, Design Engineer TVSS

Date: 26 July 2006






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1 P

URPOSE

This docum ent provides inform ation about the selection of protective devices for surge

protection equipm ent. Issues covered include device type, current rating and connectionm ethods.

2 I

NTRODUCTION

The fusing of surge protective devices (SPD s) is a contentious issue and in m any cases has

been handled badly by S PD specifiers and installers because they don’t necessarily

understand the special requirem ents of fusing SPD s. The fact that S PD s can and do fail is a

‘given’and w hilst w e all agree thatsome form of protection is required, there is little

agreem ent on the m ethods used. U sually, the m anufacturer only m akes a m inim al

recom m endation - usually a current rating and/or standards reference. A t best, the installer

follow s these instructions A N D actually consults a standard. U nfortunately, for ad-hoc

installations, little thought is usually given. This m eans that you end up either w ith no over-current protector, an appropriate one or an inappropriate one.

• W ith no over-current protector, substantial dam age m ay occur to the installation under

cases of overvoltage, extrem e surge current or the SPD m erely com ing to the end of

it’s useful life. This w ill alm ost certainly result in localised loss of pow er to the

installation. This also contravenes m ost, if not all electrical codes anyw ay, so it is

unlikely in practice. There are m any circum stances though, w hen a separate SPD

protector is not required –read on.

• A n appropriate over-current protector w ill w ithstand expected surge events w hilst

releasing under true fault conditions to lim it dam age to the S PD only.

• A n inappropriate solution m ay protect the installation but cause a pow er outage. It m ay

allow dam age to occur by disconnecting the S PD under expected surge conditions. A t

w orst, it w ill never be able to protect the SPD because the line’s rating is so low that

the m ain fuses w ill disconnect the supply first. In som e circum stances (low -current,

highly-exposed installations), this is unavoidable –read on.

W hat is also im portant is to understand is w hy SPD s fail in the first place. The truth is, m ost

SPD s w ill last for m any years. The things that cause sudden failure (in descending order of

occurrence) are:

1. External supply faults such as overvoltage (faulty transform er, M V lines across LV

lines etc)

2. Local supply faults (broken or ungrounded neutral).

3. W rongly-selected SPD voltage.4. A surge in excess of the S PD ’s rating.

5. A fault w ithin the S PD itself (usually tested at factory).

The first 2 (and m ain) causes are unavoidable, and w ill m ost likely cause further dam age to

other parts of the installation. N um ber 3 can be avoided by selecting SPD s w ith adequate

M C O V for the service. N um ber 4 is rare and generally only occurs w hen a direct strike occurs

to the supply infrastructure. A gain, as in 1 & 2, substantial dam age m ay also occur to other

parts of the installation. N um ber 5 is usually dealt with by factory testing and is unlikely to

occur in the field. Exposure to heat and m oisture can be a cause of early failure in SPD s that

haven’t been stored or installed correctly.






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U nderstanding these points, it can be seen that m ost faults that w ould dam age an S PD w ould

also cause other dam age to an installation. For this reason, it is usually accepted that it w ould

alw ays be safer to disconnect the site from the source of the fault than to sim ply disconnect

the SPD and leave the site pow ered. In som e cases (see below ), this is not acceptable, but for

m ost it is. Just bear in m ind that the client is expecting that their investm ent w ill be protectedunder the m ajority of reasonable events that m ay occur.

A t the other end of the scale, it is pointless pretending that AN Y solution is 100% capable of

protecting a site against unforseen events, regardless of cost. To this end, A S 1768 and

various EN standards recom m end a risk-m anagem ent principle to be applied, w hich is

adm irable IF the system designer know s A LL the num bers. The truth is, m ost system s are

sim ply over-engineered because the designer sim ply doesn’t have the required inform ation,

doesn’t understand the problem , or fears the client’s litigious nature!

3 T

HE NOT

SO

SIMPLE PROBLEM

S urely, an issue as sim ple as over-current protection requires little thought these days? W henit com es to protecting SPD s, som e different rules apply, com pared to other applications.

For exam ple, interrupt rating is very im portant w hen selecting any protective device but w ith

S PD protection, the ability to w ithstand com parable currents (15-100kA ) for short periods of

tim e (< 1m s) W ITH O U T releasing is m ore im portant.

In norm al operation, S PD s draw very little pow er –few exceed 250m A , except devices that

incorporate filter com ponents. This m eans that the installation of an SPD generally does not

add to m axim um dem and, allow ing the fuses to be sized close to the supply rating.

A protective device for an SPD m ust:

• H ave an interrupt rating suitable for the application. A s SPD s are norm ally connected

im m ediately after the m ain isolator, they m ay have to be able to interrupt 15-100kA

under fault conditions.

• N ot release under expected surge conditions. This w ill effectively disconnect the SPD

from the system , rem oving protection from the site. Surge ratings range from 3kA for a

sub-board to 100kA + for a highly exposed or im portant site.

• W hen required to release, they m ust do so quickly. Tim e delay m ethods increase the

likelihood of dam age occurring and of the fault current m itigating to a higher level, such

as a transform er fuse.

Then there is the inescapable fact that a system ’s ability to be protected w ithout loss of

supply is directly proportional to its supply rating. W hat this m eans is that the higher theservice rating, the easier it is to protect. In sizing SPD system s, m any designers forget this. It

is not a sim ply a m atter of site exposure either. For exam ple, a highly exposed 400A service

w ould be easier to protect than a m edium exposure 40A service. It all com es dow n to how

m uch protection the client requires –so m uch it hurts, or som ew hat less?

It therefore m atters a great deal, w hether or not the client can accept a pow er outage in

preference to surge protection. If not, one m ust be careful not to m ake inappropriate decisions

regarding fusing.






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3.1 Fuses vs. Circuit Breakers

O ne recom m endation m ade by m any surge practicians is to avoid circuit breakers (C B s)

altogether. This m akes sense for a num ber of reasons. The m ain one being that there is no

standard for C B perform ance under surge conditions, except for transm ission line equipm ent.

This m eans that certain ‘features’in particular brands m ay m ake the device either m ore or lesssuitable for surge applications. O ther reasons include:

• If the trip coil has appropriate characteristics, it w ill operate under surge conditions,

releasing the C B . This w ill leave the site unprotected or, if a supply C B , unpow ered. It

also possesses a finite inductance, adding to the let-through voltage rating of the S PD .

• M ost C B s include a ‘tortuous path’to prevent dam age under high-current release

conditions by reducing the chance of destructive flashover inside the device.

U nfortunately, to a surge, the tortuous path represents a very short, high im pedance

part of the surge path. In certain cases, the surge energy sim ply jum ps across the

path. This m ay not alw ays dam age the C B im m ediately but w ill definitely shorten its

life and m ost likely cause it to release after the surge event.• If a C B has tripped, it is all too easy just to reset it. The result can be disastrous if the

fault hasn’t been cleared beforehand. If the C B has tripped due to SPD m alfunction,

the SPD m ay ignite or the fault m ay m itigate to the prim ary disconnector, rem oving

pow er from the site. In extrem e cases, the operator m ay be injured if the C B is

adjacent to the S PD . A s fuses require replacem ent, there is m ore chance that the fault

w ill be investigated (because you just can’t reset it again afterw ards, like a C B ) or a

service call to the SPD supplier w ill be m ade, avoiding the consequences of a ‘quick

fix’.

• C ost-w ise, C B s have a lim ited interrupt capacity. Since prim ary surge protection m ust

be fitted at the point-of-entry, an interrupt rating of 50kA m ay be required w hich is a

rather expensive C B , com pared to H R C fuses.

For these reasons, it is assum ed that SPD system designers and specifiers w ill use H RC

fuses for SPD protection. A s Eaton is a m anufacturer and supplier of both C B s and fuses,

w e could m ake m ore m oney selling C B s, but they are not the appropriate choice. In A LL

cases, gG /gL fuses are specified. D o not use delay or sem iconductor types.

3.2 FUSE SURGE RATINGS

M any engineers assum e that fuses are not greatly affected by 8/20us surges. A s long as the

interrupt rating is adequate, they don’t think too m uch about the fuse current rating and sim ply

apportion the fuse rating to be som ew hat less than the supply fuses. In m any cases, this

w orks w ell, but in others, it is courting disaster. The reason is that the service fuse rating

determ ines the m axim um surge current that m ay be conducted through the system .

W e have tested m any different values and styles of H RC fuses, and although different styles

of H RC fuse have slightly different characteristics, m ost seem to follow a sim ilar trend, at

least for fuses rated up to 690V . This m eans that for a given A C current rating, regardless of

the actual brand or style of H R C fuse used, there is a m axim um surge current that m ay be

carried by the fuse w ithout rupture. S ee Figure 1 and Table 1 follow ing.






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Figure 1.

A n 80A N H -style fuse after exposure to a 50kA 8/20us pulse. A s can be seen, the fuse

literally exploded, covering the inside of the test cham ber w ith filler m aterial. The fuse at rear

w as not connected at the tim e. The rating eventually attributed to this fuse w as 25kA .

Service fuse rating

(A )

S ervice fuse surge

rating

(kA )

M axim um TVS S fuse

(A )

(Service-33% )

C orresponding m ax.

TV S S fuse surge

rating (kA )

32 7.5 20 < 5

40 15 25 5

50 17.5 32 7.5

63 20 40 15

80 25 50 17.5

100 40 63 20

125 60 80 30160 65 100 40

200 100 125 60

250 120 160 80

315 130 200 100

400 200 250 120

500 240 315 160

630 260 400 200

Table 1 –Fuse ratings vs. surge ratings






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From table 1 it can be plainly seen that it is not possible to conduct C ategory C surges

through fuses rated below 40A . A s C ategory C protection (15kA 8/20us) is generally

considered the m inim um for system protection, a 63A service is the low est-rated service that

could be rated as fit for exposure. In this case, one has to question the purpose of specifying

highly rated SPD s on sm all or rem ote sites, beyond sim ply ensuring long life.

N ote that the surge current is not presented as either In or Im ax. The 15kA C ategory C level

given is assum ed to be In, requiring an S PD rated at 40kA Im ax. If the designer w anted a 40kA

In rating, the SPD fuse w ould be required to be rated at 100A . This im plies a service rating of

150A or m ore. If the service w as rated at only 63A , the supply fuses alone w ill only w ithstand

20kA ! Even w ith the SPD protected by a 40A SPD fuse, it is likely that any surge great

enough to rupture the 40A S PD fuse (> 15kA ) w ould also be likely to rupture the 63A supply

fuse (~ 20kA ) as w ell.

The ‘TV SS fuse rating’is sim ply the supply fuse less 33% , to allow som e overhead. This level

should only be used in system s w ith low average dem and. For higher dem and levels, a low er

percentage of the supply rating should be apportioned to the SPD , although this becom es

problem atical for supplies below 200A .

TIP From testing experience, the sm aller the fuse package, the better its ability to w ithstand

surges. B S88-style or cylindrical fuses are preferred over N H -style or ‘lock’fuses. If you

have a choice, tag-style B S88 fuses are m ore reliable and offer a low er voltage drop under

surge conditions. They are also less likely to explode or physically rupture, even under extrem e

surge conditions. The figures in table 1 are true for m ost types dow n to 63A . B elow 63A , the

data only applies to B S88-style fuses but should be representative of m ost types. For N H

types, de-rate the surge rating by a further 25% below 63A .

3.3 What now?

It really com es dow n to how im portant it is for the site to rem ain active follow ing a m ajor surge

event and the likelihood of that event occurring. In som e cases, supply disconnection is not

acceptable:

• The incom e generated by the system m ay far outw eigh the equipm ent repair costs and

running the system unprotected during peak tim es m ay be preferable to shutting it

dow n for repair.

• Labour costs can be prohibitive. Sending a com pany technician to a rem ote site to

replace sm all pieces of equipm ent in a tim ely m anner is cheaper than having to send a

team of qualified people to the sam e site in an em ergency.

• The site m ay process m aterials for m anufacture and the loss of pow er m ay causeexpensive w astage.

• Public buildings, hospitals and hotels also require special treatm ent as law s generally

require evacuation follow ing loss of pow er.

Sites that w ill tolerate loss of pow er over protection are generally those that have large

am ounts of sensitive and expensive equipm ent and have been designed to w ithstand extended

periods of pow er loss. Exam ples are sites w ith com bined U PS and generator backup, such as

call centres, hospital operating theatres, telephone exchanges and large netw ork server

system s.






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S ites that are non-pow er loss tolerant m ust be rated in excess of 63A , just to m eet a C at C

rating (80A for a sm all m argin of safety). To m eet a 15kA In level (Im ax = 40kA ), the site m ust

be rated at 150A or m ore. The follow ing table lists som e points applicable to pow er loss

tolerant and non-pow er loss tolerant sites:

Non power loss tolerant Power loss tolerant

N o backup or backup tim e less than repair

tim e.

Extended backup capability such as U PS w ith

extended runtim e or generator backup.

R aw m aterials subject to w astage Processes not left running or allow ed to stop.

C ostly re-start procedure Sim ple or autom atic re-start procedure.

Pow er loss w ould involve evacuation of

personnel.

N on-populated areas or technical operators

trained in correct procedures.

B usiness incom e is directly related to

provision of pow er (m ovie theatre etc)

B usiness incom e not directly related to

provision of pow er (shop, outdoor etc)

Pow er loss m ay involve loss of property or life

(m ulti-storey building, outdoor lightingsystem , parking station boom gates)

Site is environm entally benign and poses no

threat under pow er loss conditions (dom esticdw elling).

M edium to high pow er sites (> 80A ) Low pow er sites (< 80A )

A site doesn’t have to posses all of these qualities –it m ay just be one, depending upon the

client’s needs. If the client is convinced that the site is non-pow er loss tolerant, rem ind them

that pow er outages do occur and ensure that the rest of the site is pow er loss tolerant

regardless of faults related to a faulty S PD . The last thing one needs is an irate client w ho

blam es his pow er outage on your recom m ended equipm ent!

4 SOLUTIONS

O nce this decision is m ade, the rest of process is sim ple. The choice to the designer isw hether to protect the SPD at or below the service rating.

4.1 Non power loss tolerant sites

For non-pow er loss tolerant sites, assum e that surge currents are below the level that w ill

rupture the supply fuses (see Table 1). Then, determ ine the appropriately-rated S PD fuse and

ensure that the S PD specified is rated for an In level at least equal to the expected surge

current.:

• The m axim um S PD fuse rating w ill alw ays be at least 33% below the supply fuse rating

and m ay be m uch low er in value for higher-pow ered installations.

• Low pow er sites and sites w ith high average load are harder to protect than those w ith

interm ittent loads. This is because the supply fuses have less ‘headroom ’under norm aloperation. In som e cases, it m ay be necessary to connect the SPD fuses directly to the

supply, instead of the secondary side of the supply fuses. This preserves the service

fuses because fuses rated at 40 or 50A cannot conduct m ore than 10-15kA w ithout

rupture. B y connecting the SPD fuses directly to the supply, the service and SPD

fuses can be the sam e value. For exam ple, a 100A service using series fusing could

only support a surge rating of 20kA (63A SPD fuse) w hereas connecting the S PD

fuses directly to the supply gives a w ithstand rating of 40kA . This corresponds to a

device rated at In= 40kA , I m ax= 100kA , a relatively rugged S PD specification.

• The greater the difference betw een the supply fuse rating and the S PD fuse rating, the

better coordination and the less chance that SPD failure w ill cause pow er loss. W ith

higher-pow ered sites (surge rating of supply fuse is 2-3 tim es the intended S PD rating






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or m ore), little consideration is required as the S PD and supply fuses w ill coordinate

correctly and S PD -induced pow er loss is unlikely. For exam ple, a 1200A service,

protected to a 100kA level (200A SPD fuse)

Referring to Table 1, select the SPD fuse appropriate to the service and no less than w hatw ill provide reliable operation at the desired surge current.

4.2 Power loss tolerant sites

For pow er loss tolerant sites, it is im portant to m axim ise surge current tolerance, to provide

the m axim um level of protection. S uch sites usually have higher surge current w ithstand

requirem ents and therefore, m ore perform ance is required from the fuse.

• In cases w here the fuse required for SPD protection is higher in value than the supply

fuses, the SPD fuses m ay be om itted altogether, utilizing the supply fuses for both

service protection and SPD protection. O N LY D O TH IS IF TH E SU PPLY FU SES A RE

gG /gL HR C TYPES.

•

If the separate S PD fuse is om itted, the SPD m ay be w ired in ‘Kelvin’configuration,w hich prevents losses in cabling from increasing the let-through voltage under surge

conditions. In ‘Kelvin’m ode, cables to the SPD m ay be relatively long w ithout

detrim ent to the protection level.

• W ith low pow er sites, it m ay be necessary to connect the SPD fuses directly to the

supply, instead of the secondary side of the supply fuses. This preserves the service

fuses because fuses rated at 32 or 40A cannot conduct m ore than 10-15kA w ithout

rupture. It also m eans that the line fuses have m ore chance of surviving a surge as

they are not conducting surge currents. This m eans that the service and SPD fuses

can be the sam e value, as the pow er consum ption of an SPD in norm al operation is

negligible.

R eferring to Table 1, select the fuse according to the supply current rating or, for m edium

pow ered system s, use Table 2 below :

SPD per-m ode rating M inim um SPD fuse size (A )

20kA 63

40kA 100

80kA 200

120kA 250

200kA 400

Table 2 –Fuse ratings vs. surge ratings

5 C

ONCLUSION

The inform ation presented here should prove useful to engineering consultants, estim ators and

system designers. W hilst possibly appearing overly sim plified, there is no point providing an

intense level of descriptive detail w hen it is not required.

If installations are designed in accordance w ith the inform ation presented here, a cost-

effective, safe and reliable TV SS system should result.

Jon P oint

Documents

Fuse Selection Surge Applications