Fuses and Circuit Protection

Every circuit can fail: a wire rubs through and touches metal, a part shorts inside, or a spike of voltage arrives from outside. Circuit protection parts are there to make that failure safe. This article explains fuses, resettable polyfuses, TVS diodes and circuit breakers: how they work, how to read and choose them, how to test them, and how to stay safe.

1. A short recap

• Current is the flow of charge, measured in amperes (A); voltage is the push behind it, measured in volts (V).
• Current through any resistance makes heat. The power turned into heat is P = I²R, so doubling the current makes four times the heat (recap).
• A short circuit is an unintended path with almost no resistance, so a very large current flows. Every wire has a safe current, and a fuse must open before the wire overheats (recap).
• A real source has internal resistance, which sets the most current it can push into a short (recap).
• A diode driven hard enough in reverse breaks down and conducts; Zener and TVS diodes do this safely (recap).
• Breaking a current can draw an arc, a spark that keeps conducting through air; DC arcs are harder to stop (recap).

2. Why circuits need protection

Two kinds of trouble cause most damage. The first is overcurrent: more current than the circuit was designed for. A mild, long-lasting overcurrent, such as too many heaters on one outlet, is called an overload. A sudden, huge overcurrent through a short circuit is called a fault. Either way, wires heat up, insulation can melt, and fires can start. Overcurrent devices exist mainly to stop conductors from overheating.^[1]

The second is overvoltage: a brief spike of voltage, called a transient, far above the normal supply. Transients come from switching off motors and relays, from static discharges, and from lightning, even when it strikes some distance away.^[2] A transient may last only microseconds but can still destroy chips.

A fuse or circuit breaker watches the current and opens the circuit when it is too high; a polyfuse is a fuse that resets itself. A TVS diode watches the voltage and diverts spikes.

3. How a fuse works

A fuse is a short wire or metal strip, the element, designed to melt when the current is too high. It is connected in series with what it protects, so when it opens, all the current in that path stops.^[1] Littelfuse, a large fuse maker, calls fuses "intentional weak links" in the circuit.^[3]

The mechanism is just heating. The element has a small resistance, so the current heats it by I²R. At normal current the heat escapes as fast as it is made; above the rating it builds up until the element melts. Because heat grows with the square of the current, a large overcurrent melts the element much faster than a small one.^[3]

Melting is not the end. As the element parts, the voltage across the new gap can draw an arc that keeps current flowing. The fuse has cleared only when the arc goes out. The time until melting is the pre-arcing time, the arc then lasts for the arcing time, and the two add up to the total operating time.^[4] That is why a fuse has a voltage rating as well as a current rating: the gap must be long enough, and the body strong enough, to put out the arc at that voltage.^[1]​^[3]

The energy needed to melt an element is given as its melting I²t, current squared times time, in ampere-squared seconds (A²s). It is measured with pulses short enough (about 8 ms or less) that no heat escapes.^[3] A Littelfuse 217-series 2 A glass fuse needs about 5.7 A²s; its 0.1 A version needs only 0.0026 A²s.^[5]

Fuses go on the "hot" side of a circuit, the side that is not connected to ground. A blown fuse in the grounded return wire would leave the load still connected to the live supply, ready to shock.^[1]

4. Fast and slow fuses: the time-current curve

A fuse does not blow at an exact current. How long it takes depends on how far the current is above the rating, and this is shown on a time-current curve: current along the bottom, time to melt up the side, both on stretched (logarithmic) scales.^[3] The curves are averages, so the guaranteed limits matter more.^[3]

The 217 series is a fast-acting fuse. Its data sheet guarantees, for ratings from 0.125 to 6.3 A: at 210% of the rating it opens within 30 minutes; at 275% within 2 s; at 400% within 0.3 s; and at 1000% (ten times the rating) within 0.02 s.^[5] Near its rating it is slow: a fuse is not a precise current limit.

Some loads draw a large inrush current when switched on. A motor can draw up to ten times its normal current while starting, and a fast fuse would blow every time.^[1] A power supply's big smoothing capacitor causes a similar surge.^[3] A time-lag fuse, also called slow-blow, has extra thermal mass in its element, so it heats more slowly and rides through a brief surge but still opens on a long overload.^[1]​^[3] (Motors get their own article: 20, Motors.)

5. Common types of fuse

• Glass cartridge. A glass tube with metal end caps; you can see the element and whether it has blown. The 5 × 20 mm size (5 mm across, 20 mm long) is the common international size; the 217 series has a glass body and nickel-plated brass caps.^[5] Older North American sizes have names such as 3AG ("Automobile Glass").^[3]
• Ceramic cartridge. Same shape with an opaque ceramic tube, often filled with a material that helps put out the arc. These interrupt far larger fault currents than glass fuses.^[4]
• Blade fuses. The plug-in fuses in cars: a colored plastic body with two flat metal prongs. Littelfuse's ATO type is rated 32 V DC, has a see-through body so you can see a blown element, and has test points on top for checking it in place.^[6]
• Surface-mount chip fuses. Tiny rectangular fuses soldered directly onto circuit boards, from about 1 mm long upward.^[3]
• Polyfuses (PPTC). Resettable devices, covered in section 8.

6. Units and markings

Three numbers describe any fuse:

• Current rating (amperes): the current the fuse is designed to carry under standard test conditions.^[3]
• Voltage rating (volts): the highest voltage at which it is guaranteed to clear safely. It may be used at any lower voltage. Common ratings for small fuses are 32, 63, 125, 250 and 600 V.^[3]
• Breaking capacity, also called the interrupting rating (amperes): the largest current the fuse can safely interrupt at its rated voltage without bursting.^[3] It ranges from 35 A for some 5 × 20 mm glass fuses to 200,000 A for large industrial ones.^[3] An ATO blade fuse is rated to interrupt 1,000 A at 32 V DC.^[6]

Fuses made to the international standard IEC 60127 are marked with a short code on an end cap, in this order: a letter for the speed, the current rating, a letter for the breaking capacity, and the voltage rating.^[4] The speed letters are FF (very quick acting), F (quick acting), M (medium time-lag), T (time-lag) and TT (long time-lag). The breaking-capacity letters are L (low: typically a glass fuse, 35 A or ten times the rating, whichever is greater, at 250 V AC) and H (high: typically ceramic, often with an arc-quenching filling, 1,500 A at 250 V AC).^[4] Small currents are often written in milliamperes, such as 500 mA.

North American fuses follow a different standard (UL 248), which tests them differently. For example, a UL-listed fuse must open within an hour at 135% of its rating, while an IEC quick-acting cartridge fuse must not open at 150%.^[3] So UL and IEC fuses with the same label numbers are not necessarily interchangeable.^[4] Breaking capacities are usually given for AC; the DC figure is considerably lower,^[4] because a DC arc never passes through zero the way an AC one does (recap).

7. Choosing a fuse

Littelfuse's selection checklist comes down to a few questions:^[3]

1. Normal current, with margin. Run a fuse at no more than 75% of its rating at 25 °C, or it may age and blow for no good reason (a nuisance opening). Littelfuse's example: a 1.5 A load needs 1.5 ÷ 0.75 = 2 A.
2. Temperature. A hot enclosure lowers the current a fuse can carry. In the same example, a time-lag fuse allowed only 80% at 75 °C needs 1.5 ÷ (0.75 × 0.80) = 2.5 A.
3. Voltage. The fuse's voltage rating must be at least the circuit voltage.
4. Fault current. The breaking capacity must be at least the largest current the supply could push into a short.
5. Surges. If the load has inrush, choose a time-lag fuse, and check the surge's I²t against the fuse's melting I²t; a good rule is to keep the surge below 20% of it.

Worked example. A project runs from a 12 V lead-acid battery and draws 1.5 A, with a capacitor on its input that causes a short surge at switch-on. The steps above give a 2 A fuse rated at least 12 V, checked against the surge. Suppose the battery's internal resistance is 0.02 Ω. Into a dead short it could drive about 12 ÷ 0.02 = 600 A (calculated, for this assumed resistance). A glass fuse with a 35 A breaking capacity could shatter or keep arcing; an automotive blade fuse, rated to break 1,000 A at 32 V DC, is the right family.^[6] A gray 2 A ATO fuse has a typical melting I²t of 1.4 A²s,^[6] so by the 20% rule the switch-on surge should stay below about 0.28 A²s (calculated). The fuse goes close to the battery's positive terminal, to protect the wire too.

Small fuses also have resistance. A 0.1 A glass fuse measures about 5.7 Ω cold, which drops noticeable voltage in a low-voltage circuit; a 2 A one is about 0.04 Ω.^[5]​^[3]

8. Polyfuses: fuses that reset

A polyfuse, or PPTC (polymeric positive temperature coefficient) resettable fuse, is made of a plastic (polymer) filled with tiny particles of conductive carbon black. When cool, the polymer is partly crystalline and the carbon particles touch, forming conducting chains; the resistance is low, from a few milliohms to a few ohms.^[7]​^[8] "Positive temperature coefficient" means its resistance rises with temperature, like a PTC thermistor (recap).

Too much current heats it by I²R. At roughly 90 to 160 °C the polymer changes from partly crystalline to amorphous (disordered) and expands, pulling the carbon chains apart. The resistance jumps by several orders of magnitude, a thousand times or more, and the device is tripped.^[8] A small current still flows, and its heat keeps the device tripped while the fault and power remain.^[9] Remove the power and it cools, recrystallizes and resets.^[9] After a trip the resistance is typically about 50% higher than when new, a change that does not keep growing with further trips.^[8]

Polyfuse data sheets use their own terms:^[7]

• I_hold: the most current it carries without tripping.
• I_trip: the least current that will trip it.
• V_max and I_max: the highest voltage and fault current it can withstand.

For example, Bourns's MF-PSMF050X, a chip only 2 mm long, holds 0.50 A and trips at 1.00 A at 23 °C, works up to 6 V, survives a 40 A fault, and trips within 0.1 s at 8 A.^[10] Between I_hold and I_trip it may or may not trip. Heat matters even more than for a fuse: its hold current falls to 0.23 A at 85 °C.^[10]

Polyfuses suit faults that may be temporary, such as a shorted USB cable. They are slower than a fast fuse: the MF-PSMF050X may take 0.1 s at 16 times its hold current,^[10] where a 217 fuse opens within 0.02 s at 10 times its rating.^[5] Each type also has a maximum voltage, only 6 V for that chip.^[10] Many lithium-ion cells also contain a PTC device, alongside the battery's other protection (recap).

9. TVS diodes: clamping voltage spikes

A fuse cannot stop a voltage spike: it is far too slow, and a spike may do its damage without much current. Overvoltage needs a different idea. A clamp is a part connected across the supply or signal line, in parallel with the circuit, that does nothing at normal voltage but conducts heavily when the voltage rises too far, holding the voltage down and steering the surge current away to ground.^[11]

A TVS (transient voltage suppression) diode is such a clamp. It is a silicon diode built to break down in reverse, like a Zener, but with a much larger junction so it can absorb high peak energy; a Zener is designed to hold a steady voltage, a TVS to survive a surge.^[11] Below breakdown it leaks only a tiny current; above it, it conducts as hard as needed to hold the voltage near its clamping level, switching on in under a nanosecond.^[11] Afterwards it returns to its off state. TVS diodes appeared briefly in the Diodes article (recap).

The key numbers on a TVS data sheet are:^[12]

• V_R, the stand-off voltage: the highest voltage it can sit at without conducting. Choose it above your circuit's highest normal voltage.
• V_BR, the breakdown voltage: where it starts to conduct, measured at a small test current such as 1 mA.
• V_C, the clamping voltage: the highest voltage across it while carrying its rated peak current. This is what your circuit actually sees, so it must be below what the circuit can survive.
• I_PP and P_PPM: the peak pulse current and power it can absorb in a standard test pulse.

Worked example. For a 24 V supply, a Littelfuse application note suggests an SMBJ28A or SMBJ30A.^[2] The SMBJ28A's numbers are: stand-off 28 V, comfortably above 24 V; breakdown 31.1 to 34.4 V; and clamping at most 45.4 V while carrying 13.3 A.^[12] The SMBJ family is rated for 600 W peak in a standard surge that rises in 10 µs and decays to half in 1,000 µs (the "10/1000 µs" pulse); 45.4 V × 13.3 A is about 600 W (calculated).^[12] Notice the gap: the clamp holds the line far below a spike's hundreds of volts but well above 24 V, so whatever it protects must briefly tolerate about 45 V.

TVS diodes come in two kinds. A unidirectional one clamps positive spikes and conducts like an ordinary diode for negative ones; it is marked with a cathode band, and its cathode goes to the positive line. A bidirectional one (suffix "CA" in this family) clamps both ways, for AC or signals that swing both ways.^[12] Place a TVS as close as possible to the connector where spikes enter, with short connections.^[11]

Spikes come from switching off motors, relays and transformers (hundreds of volts), static discharge (up to 35 kV from walking on carpet), and lightning a mile away (70 V induced in cables).^[2] In a car, a load dump, the battery disconnecting while the alternator is charging, can push the 12 V system to 42 V.^[13] Coils fight any change in current, which is where their spikes come from (recap).

Other clamps. A metal-oxide varistor (MOV), made of zinc oxide grains, is cheap and absorbs large energy, so it is common on AC power lines, but it clamps higher and ages with use^[11] (recap). A gas discharge tube (GDT) conducts through ionized gas; it carries huge surges but turns on slowly.^[11] Designs often combine them with a TVS.^[2]

10. How a TVS differs from a fuse

The two work as a team. A TVS absorbs short spikes, but a long overvoltage, such as a wrong power supply, would make it carry a large current until it overheats. The SMBJ data sheet says the typical failure is a short circuit.^[12] With a fuse ahead of it, a TVS that conducts for too long, or fails short, draws enough current to blow the fuse and disconnect the supply. Littelfuse's application examples pair TVS diodes with fuses in just this way.^[2]

This front end combines earlier ideas: the series diode blocks a battery connected backward (recap), and the voltage regulator after it is a part the TVS is guarding (recap).

11. Circuit breakers

A circuit breaker is a switch that opens itself automatically on overcurrent and can then be closed again by moving its lever.^[1] Inside, a latch holds spring-loaded contacts closed; a trip unit senses the current and releases the latch. The common type in homes and small equipment is thermal-magnetic, with two trip elements for the two kinds of trouble.^[14]

• Thermal trip, for overloads. The current passes through a bimetal strip, two metals that expand at different rates bonded together. Heat bends it, and when it has bent far enough it releases the latch. More current trips it sooner, like a fuse. It also feels the surrounding temperature; Schneider calibrates the thermal elements of its molded-case breakers for a 40 °C surrounding temperature.^[14]
• Magnetic trip, for short circuits. The current also passes through an electromagnet. A short-circuit current makes its field strong enough to pull in an iron armature that releases the latch at once, "with no intentional delay".^[14] ABB says this part responds within milliseconds.^[15] It is the same coil-pulls-iron idea as a relay (recap).

A breaker's trip curve combines the two: a sloping thermal part for overloads and a near-vertical magnetic step for short circuits. Small modular breakers built to IEC standards, called miniature circuit breakers (MCBs), are graded by where that step lies: type B trips magnetically at 3 to 5 times the rated current, type C at 5 to 10 times, and type D at 10 to 20 times. A D-curve breaker rides through the inrush of motors and transformers that would trip a B-curve one.^[15]

When a breaker trips, its handle usually moves to a middle position. To reset it, push the handle fully to OFF, then back to ON.^[14]​^[16] Home panels also hold special breakers: an AFCI watches for dangerous arcing and a GFCI for current leaking to ground through a person (recap).

12. Why a breaker is not a fuse

Both stop overcurrent, and a home panel may use either,^[16] but they differ:

• One-shot or reusable. A fuse destroys itself to open the circuit and must be replaced; a breaker opens and can be reset.^[1]​^[16] The upside: every replacement is a fresh, untouched part.
• Material or mechanism. A fuse has no moving parts. A breaker relies on a latch, springs and contacts, so Schneider suggests exercising the trip mechanism of its molded-case breakers at least once a year with their push-to-trip button.^[14]
• Switch or not. A breaker can be switched by hand to make a circuit safe to work on. A fuse holder is not a switch: change fuses only with the power off.^[3]​^[4]
• Speed on big faults. On a severe short, a fast fuse can melt before the current reaches its peak, limiting the "let-through" energy.^[3] Very fast grades protect semiconductors.^[1]​^[4]
• Temptation. A fuse invites the dangerous "fix" of a bigger fuse; a breaker invites the dangerous habit of resetting it again and again. Both hide a fault that needs finding.^[17]

Neither one protects people from electric shock. They open only for currents large enough to overheat wiring,^[1] far more than the 10 to 20 thousandths of an ampere that can make muscles clamp so a person cannot let go (recap). Shock protection is the GFCI's job.

13. How protection behaves in a circuit

• Series for current, parallel for voltage. Fuses, polyfuses and breakers go in series, and a fuse in one parallel branch protects only that branch.^[1] Clamps go across the line.
• Protect the wire first. Size the fuse for the smallest wire it feeds, and put it where the power enters, so that every wire after it is protected.^[1]
• Protection at several levels. A house breaker protects the wall wiring, an appliance fuse its insides, a polyfuse a port; ideally the device nearest the fault opens first.
• Holders and heat count. Littelfuse suggests running fuse holders at no more than 60% of their own rating, because the clip contacts make heat too.^[3]
• Mind the drop. A polyfuse or a small fuse adds resistance, which matters in low-voltage, high-current circuits.^[3]​^[10]

14. Testing

Fuses. Fluke's procedure: turn off and disconnect the power, remove the fuse, set the multimeter to continuity (beep) or resistance, and touch the probes together first to check the meter. Then touch the probes to the two ends of the fuse. A beep or a reading near 0 Ω means it is good; no beep or "OL" (open) means it is blown. Then look at it: a broken element or a dark, burnt glass also means it has blown.^[18]

Blade fuses with top test points can be checked in place.^[6] In a live low-voltage circuit, a good fuse shows almost no voltage across it and a blown one the full supply voltage; never try this on mains equipment unless trained (recap).

Polyfuses. With the power off and the device cool, measure its resistance and compare it with the data sheet's maximum after tripping (R1_max, 0.9 Ω for the MF-PSMF050X).^[10] Far higher means still hot or damaged.

TVS diodes. A healthy unidirectional TVS tests like a diode on the meter's diode setting: a forward drop one way, open the other (recap). A TVS that reads near zero both ways has failed short, the usual way they fail.^[12]

Breakers. In a house, use only the handle and any test button. A breaker that trips again straight away, or trips often, means a fault or overload that a qualified electrician should find.^[16]​^[17]

15. Everyday uses

• Cars are full of blade fuses, one for each circuit.^[6]
• Homes have breakers (or, in houses wired before about 1965, fuses) in the service panel for every circuit, plus AFCI and GFCI protection.^[16]
• Appliances and power supplies have a fuse on the incoming line, often a 5 × 20 mm cartridge.^[5]
• Multimeters fuse their current ranges, so a mistaken measurement blows a fuse rather than the meter (recap).
• USB and HDMI ports, phone batteries and game consoles use polyfuses.^[10]
• Data lines, chargers and car electronics use TVS diodes against static, switching spikes and lightning.^[12]​^[2] Surge-protected power strips use MOVs on the AC line.^[11]

16. Safety

• Never replace a fuse with a higher rating. ESFI calls it "a very dangerous practice and a serious fire hazard", and gives the example of a 30 A fuse in a 20 A circuit.^[16]​^[17] The wire would overheat before the bigger fuse opened.
• Never bypass protection. Foil, wire, a coin behind a fuse or a taped-down breaker leaves the circuit with no weak link at all, so the weakest point becomes the wiring itself.
• Replace like with like: the same current, voltage, speed (fast or time-lag) and breaking capacity, and the same size. A glass fuse where a ceramic one belongs may burst on a heavy fault.^[4]
• Find the cause. A fuse that blows again at once, or a breaker that keeps tripping, has found a fault. For a home breaker, call a qualified electrician.^[16]
• Power off first. Switch off and unplug before changing a fuse; a fuse holder is not a switch.^[4]​^[18]
• Fuse every battery. Batteries, especially lithium and lead-acid, can deliver enormous fault currents; put a fuse close to the terminal (recap, lithium safety).
• Leave mains work to electricians. Panel and outlet wiring carries lethal voltages (recap).

The whole story in brief

• A fuse is a deliberate weak link in series: its element melts, and the arc must then be put out. It is rated in amperes, volts and breaking capacity.
• Fuses open faster at higher currents; time-lag fuses ride through inrush.
• Markings such as T3.15AL250V give speed, current, breaking capacity and voltage.
• Choose a fuse at about 133% of the normal current, at least the circuit voltage, with enough breaking capacity.
• Polyfuses rise to high resistance when hot and reset when power is removed.
• TVS diodes sit across the line and clamp spikes in under a nanosecond; a fuse ahead of them handles what they cannot.
• Breakers trip thermally on overloads and magnetically on shorts, and can be reset; a fuse cannot.
• Never fit a higher rating, never bypass protection, and always find the cause.

Coming up in this section: 18 Sensors.

Sources

All links were checked in October 2026. External links open in a new tab.

1. Tony R. Kuphaldt, Lessons In Electric Circuits, Volume I (DC), chapter 12, "Physics of Conductors and Insulators" (section "Fuses"): https://www.ibiblio.org/kuphaldt/electricCircuits/DC/DC_12.html
2. Littelfuse, "Transient Voltage Suppressors (TVS Diode) Applications Overview" application note (PDF): https://www.littelfuse.com/assetdocs/tvs-diode-overview-application-note?assetguid=26685cec-30be-4b68-b9f2-6956384c9129
3. Littelfuse, Fuseology Design Guide: A guide to selecting the right fuse for your application (2022, PDF): https://www.littelfuse.com/assetdocs/fuseology-selection-guide?assetguid=d812dff2-1c47-4dc3-bce7-07a4001ddc32
4. SCHURTER, Knowledge Base, "Fuse-Link" (explanations and standards: IEC 60127 markings, breaking capacity, I²t): https://www.schurter.com/en/knowledge/fuse-link
5. Littelfuse, 217 Series 5 × 20 mm Fast-acting Fuse data sheet (revised July 2025, PDF): https://www.littelfuse.com/assetdocs/fuse-217-datasheet?assetguid=af55be94-c42e-41b1-ad43-e070e09443fe
6. Littelfuse, ATO® Blade Fuses, Aftermarket, Rated 32 V data sheet (revised November 2022, PDF): https://www.littelfuse.com/assetdocs/littelfuse-datasheet-ato?assetguid=bba6ed2a-96e5-4b07-ab5d-0e8d267c0bd1
7. Bourns, Multifuse® Resettable Fuses: Polymeric PTC & Ceramic PTC Short Form Brochure (PDF): https://www.bourns.com/docs/technical-documents/technical-library/multifuse-pptc-resettable-fuses/publications/bourns_mf_ptc_short_form.pdf
8. Bourns, "Soldering Recommendations: Multifuse® Polymer PTC Resettable Fuses" (includes how the polymer trips and recovers, PDF): https://www.bourns.com/docs/technical-documents/technical-library/multifuse-pptc-resettable-fuses/publications/bourns_mf_solder_recom.pdf
9. Bourns, "Preventing Component Circuit Damage from Above Range Current with Reliable Overcurrent Protection" (Multifuse® white paper, PDF): https://www.bourns.com/multifuse/docs/bourns_multifuse_white_paper.pdf
10. Bourns, MF-PSMF Series PTC Resettable Fuses data sheet (PDF): https://www.bourns.com/docs/product-datasheets/mf-psmf.pdf
11. Jim Lepkowski, "An Introduction to Transient Voltage Suppression Devices" (AND8229/D), Littelfuse application note (2016, PDF): https://www.littelfuse.com/assetdocs/an-introduction-to-transient-voltage-suppression-devices?assetguid=2b1b62aa-2008-473c-a3b2-062a8381062a
12. Littelfuse, SMBJ Series 600 W surface-mount TVS diode data sheet (revised July 2025, PDF): https://www.littelfuse.com/assetdocs/tvs-diodes-smbj-series-datasheet?assetguid=ba555e99-a12d-4f72-a0b6-86b06c67171e
13. Texas Instruments, Analog Applications Journal, "Designing the front-end DC/DC conversion stage to withstand automotive transients" (SLYT707, PDF): https://www.ti.com/lit/an/slyt707/slyt707.pdf
14. Schneider Electric (Square D), Legacy Thermal-Magnetic / Magnetic Only Molded Case Circuit Breakers catalog 0601CT9101 (August 2022, PDF): https://download.se.com/files?p_Doc_Ref=0601CT9101
15. Thomas Weinmann, ABB, "What you need to know about miniature circuit breaker trip curves" (technical article, PDF): https://library.e.abb.com/public/b5acf03b2a1f42e3b08ed2fc27672c30/What%20you%20need%20to%20know%20about%20MCB%20trip%20curves.pdf
16. Electrical Safety Foundation International (ESFI), "Understanding Your Home Electrical System": https://www.esfi.org/understanding-your-home-electrical-system/
17. Electrical Safety Foundation International (ESFI), "Fuse and Breaker Breakdown": https://www.esfi.org/fuse-and-breaker-breakdown
18. Fluke, "How to Check a Fuse with a Multimeter": https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-check-fuse-with-multimeter