40 theory exam questions NEN 3140 VP — with detailed answers

This is a collection of the questions that come up most often in the theory test and the oral examination surrounding a NEN 3140 VP (re-)instruction. Each question is presented in the way it is asked, with an answer that goes beyond the bare keyword — including the reasoning behind it, because that reasoning is often what an examiner is really testing.

Legal framework and roles

1. What is low voltage? Voltage up to and including 1000 V alternating voltage (AC) or 1500 V direct voltage (DC). Everything above that is high voltage and falls under a different standard (NEN 3840). NEN 3140 itself is written exclusively for low voltage.

2. What is the safe touch voltage? 50 V AC and 120 V DC, measured under dry, normal conditions. In wet or conductive environments (for example bathrooms, outdoor areas, agricultural settings) the limit values are lower — this is why bathroom zones and outdoor spaces have extra protection requirements.

3. What does NEN 3140 govern and what does NEN 1010 govern? NEN 1010 sets requirements for the design, installation and commissioning of an installation — how it must be. NEN 3140 sets requirements for working safely on and with an existing installation or item of work equipment — how you deal with it. An installation that has been correctly installed according to NEN 1010 can still be used unsafely if NEN 3140 is not followed.

4. Where is the legal basis for all this laid down? The Arbeidsomstandighedenwet (Arbowet), article 3, obliges the employer to provide a safe working environment. The Arbeidsomstandighedenbesluit elaborates on this: article 3.4 sets requirements for electrical installations and work equipment, article 3.5 requires the written appointment of VP/VOP. NEN 3140 itself is not law, but the recognised practical standard by which an employer demonstrably complies with these statutory articles.

5. Is NEN 3140 a law? No. It is a private-law standard — the "state of the art". The Labour Inspectorate does test against it, however: if you deviate from NEN 3140, you must demonstrate that you achieve the same level of safety in another way. In practice, deviating without an equivalent alternative is a violation of the Arbowet, even though NEN 3140 itself is not literally part of the law.

6. When are you allowed to carry out live working (WOS)? Only if de-energising is not reasonably feasible — for example in a continuous process, or when de-energising itself would create a greater risk. In that case there must be a written work permit, the person carrying out the work must be a VP with specific WOS training, appropriate measures must have been taken (insulating tools, PPE, screening of adjacent parts) and a second person must be present who can intervene.

7. What is the difference between switching and isolating? Switching is interrupting the current via a control device intended for that purpose (a switch, a circuit breaker) — it makes the installation currentless, but not necessarily demonstrably de-energised and not always visibly locked. Isolating is physically and visibly establishing an isolating distance (for example by removing a fuse-link or opening a disconnector), so that re-energising without a deliberate action is impossible. For de-energised working, isolating is the required step; switching alone is insufficient.

8. When must you earth and short-circuit at low voltage? When there is a real risk of back-feed voltage or induced voltage — for example when a third party is working on the same installation at the same time, in the case of a long cable run where induction can occur, or when connected to an emergency power supply that could be switched on accidentally. For a short, isolated circuit without these risks, earthing and short-circuiting can be omitted, but you must then be able to explicitly justify that.

Risk assessment and working methods

9. What is an LMRA and how does it relate to a TRA and an RI&E? These are three risk assessments operating at a different scale. The RI&E (Risico-Inventarisatie & -Evaluatie, Risk Inventory & Evaluation) is the organisation-wide assessment of the company required by the Arbowet, recorded periodically. The TRA (Taak Risico Analyse, Task Risk Analysis) is a written assessment carried out in advance, per specific task. The LMRA (Laatste Minuut Risico Analyse, Last Minute Risk Analysis) is the short personal check that the person carrying out the work performs themselves just before the action: does the situation still match the TRA, has anything changed? In short: the RI&E is the foundation, the TRA translates that to the task, the LMRA is the final check.

10. What is "proof of proof" (proef op de proef)? Checking the measuring instrument (for example a duspol) against a known, demonstrably present voltage — both before and after the actual measurement. If the measuring instrument still functions correctly after the measurement, you know for certain that a "no voltage" reading during the measurement was reliable and not the result of a faulty measuring instrument.

11. Why do you use a voltage tester/duspol and not an ordinary multimeter to demonstrate the absence of voltage? A duspol is designed to demonstrate voltage without first having to select a range, is more robust against incorrect use, and — unlike many digital multimeters — does not have a high-impedance input that can display a phantom voltage, or fail to display one, in the event of a semiconductor fault. To demonstrate the absence of voltage, NEN-EN 50110-1 prescribes a two-pole voltage tester, not a multimeter.

12. What is the difference between operating and working? Operating is switching via a control device intended for that purpose (a switch or push-button) without any risk of contact with a live part — this is permitted, provided the device is designed for laypersons, even by a layperson. Working is any action where that risk does exist, such as opening a cabinet or replacing a component — this requires at minimum a VOP for the specifically assigned action, or a VP.

13. What are the three types of electrotechnical work according to NEN-EN 50110-1? De-energised working (the preferred method, via the 5 steps), live working (WOS, only by exception) and working in the vicinity of live parts (within the vicinity zone, but not on the live part itself).

14. What is the danger zone and what is the vicinity zone? The danger zone (DL) is the live part itself — direct contact is life-threatening and only accessible to a VP after a risk assessment. The vicinity zone (DV) is a wider distance around it — for low voltage up to 1 kV approximately 300 mm — within which additional precautionary measures already apply, even without contact. For high voltage, both distances are considerably greater.

Circuit breakers, RCDs and selectivity

15. What is the difference between a B, C and D circuit breaker? The three characteristics are thermally identical (the same rated current, the same delayed tripping under light overload), but differ in their magnetic (instantaneous) tripping current: B trips at 3–5×In, C at 5–10×In, D at 10–20×In. The higher the letter, the more inrush current (for example from a motor, transformer or LED driver) the circuit breaker tolerates without tripping unwantedly — but also the higher the required short-circuit current needed to trip within the permitted time, and thus the lower the maximum permissible loop impedance Zs.

16. What does Icn 6 kA / 10 kA on a circuit breaker mean? The rated breaking capacity: the maximum short-circuit current that the circuit breaker can safely interrupt without being damaged itself or creating a hazard. This must always be equal to or greater than the short-circuit current actually available at the point of installation — a circuit breaker with too low an Icn can itself fail in the event of a heavy short circuit.

17. What is the difference between an RCD (residual current device / aardlekschakelaar, ALS) and an RCBO (aardlekautomaat, ALA)? An RCD contains only differential protection (detects leakage current) and does not protect the cable against overload — the stated ampere rating is the through-current, not a thermal value. An RCBO combines differential protection with a circuit breaker (with B/C/D characteristic) in a single device — here the ampere rating is indeed the rated current of the integrated circuit breaker. You can recognise this on a diagram by the absence (RCD) or presence (RCBO) of a B/C/D letter next to the differential symbol.

18. What is the difference between 30 mA and 300 mA earth-leakage protection? 30 mA is personal protection: this is well below the current level at which ventricular fibrillation can occur, and is mandatory for final circuits with socket outlets up to and including 32 A for general use. 300 mA is fire protection only — this current level is already potentially fatal for a person and therefore does not protect people, only against overheating and fire caused by prolonged leakage current.

19. What are RCD types AC, A, F and B? Type AC detects only pure alternating leakage current and is no longer permitted for new installations. Type A additionally detects pulsating direct leakage current and is the standard for dwellings. Type F adds detection of high-frequency components (relevant for single-phase frequency converters). Type B also detects pure direct leakage current and is required for EV charging points and three-phase frequency converters, where a direct-current component can "blind" a type A RCD.

20. What is selectivity between protective devices? The principle that, in the event of a fault, only the protective device closest above the fault trips, while upstream protective devices remain operational — so that a fault in one circuit does not shut down the entire installation. For circuit breakers, a rule of thumb of at least a 1.6× difference in rated current between successive levels often applies. For RCDs, current selectivity alone is not enough: two ordinary 30 mA RCDs in series are not selective (both can trip at the same time) — this requires a delayed type S upstream (130–500 ms), with a fast 30 mA below it.

21. How is an RCD checked with an installation tester, and with which values? At three points: at I∆n (rated tripping current), the RCD must trip within ≤ 300 ms; at 5×I∆n within ≤ 40 ms; at ½×I∆n the RCD must specifically NOT trip — if it does, it is set too sensitively or is faulty. This is an addition to, not a replacement for, the manual test button on the device itself, which must be operated at least every six months.

Earthing systems, disconnection times and impedance

22. Which earthing systems do you know and what is the difference? TN-S (separate N and PE conductor throughout the entire installation), TN-C (combined PEN conductor), TN-C-S (PEN at the source, split into N and PE afterwards), TT (separate earthing at the source and at the installation, no direct conductive connection) and IT (no direct earth connection of the source, isolated or high-impedance earthed). TN-S is the most predictable for RCD behaviour because there is no leakage current via the PE that can disturb the measurement.

23. What are the permitted disconnection times in the event of an insulation fault? For TN systems at 230 V: final circuits up to and including 32 A must disconnect within 0.4 s, distribution circuits and fixed equipment above 32 A within 5 s. For TT systems these times are shorter (0.2 s and 1 s respectively), because the earthing path in a TT system is higher-impedance and less predictable.

24. What is the formula for the maximum permissible loop impedance Zs? Zs × Ia ≤ U₀, where Ia is the current that causes the protective device to trip within the required time (for a circuit breaker: the magnetic instantaneous tripping current) and U₀ is the rated phase-to-neutral voltage (230 V). The higher the required Ia (for example with a D circuit breaker), the lower the maximum permissible Zs — and the less room there is for a long cable or a weak earth connection.

Protective measures and classifications

25. Which protective measures against electric shock do you know? Three cumulative levels: basic protection (insulation, enclosures, distance — prevents direct contact with live parts under normal conditions), fault protection (automatic disconnection of supply, class II insulation, SELV/PELV, isolating transformer — prevents danger from indirect contact after a fault) and additional protection (30 mA RCD, equipotential bonding — the last safety net). Additional protection never replaces adequate basic and fault protection.

26. What does an IP code mean and what is the practical minimum requirement? The first digit describes protection against solid objects and dust (0–6), the second against water (0–9K). IP2X — protection against the ingress of a finger — is the practical minimum requirement for accessible parts of a distribution board: after opening a door or panel, no direct finger contact with live parts should be possible. IP44 stands for splash-water protection, IP65 for dust-tight plus resistant to water jets.

Arc flash and personal protective equipment

27. What is an arc flash (vlamboog) and why is it dangerous? An arc flash is an electrical discharge through the air caused by a short circuit or an incorrect switching action, with temperatures of up to approximately 20,000 °C — hotter than the surface of the sun. In addition to the heat, an arc flash causes a pressure wave, molten metal particles and intense light that can cause eye injury. Control measures: de-energised working wherever possible, never opening a switchgear cabinet under load unless explicitly designed for that, using insulating tools, and, where the risk is unavoidable, wearing appropriate PPE based on an arc energy calculation (cal/cm²).

28. Which PPE do you mention for work on or near electrical installations? Insulating gloves (class 00 or 0, depending on the voltage), a face shield or arc-flash visor, flame-retardant clothing, S3 safety shoes, safety glasses, insulated VDE tools (tested to 1000 V) and an insulating mat or floor plate. Which combination is mandatory follows from the risk assessment of the specific task, not from a fixed list.

29. What are the physiological threshold values of electric current through the body? Around 0.5 mA lies the perception threshold (a light tingling). Around 10 mA lies the "let-go" threshold — the point at which muscle contractions make it impossible to release a live part. From approximately 50 mA the risk of ventricular fibrillation rises sharply. The 30 mA standard for personal protection therefore lies comfortably below the fibrillation threshold, with margin for body resistance and contact duration.

Inspection, appointment and practice

30. Who determines the inspection frequency of an installation or item of work equipment, and how often is that? The employer determines this, based on their own RI&E. NEN 3140 provides a calculation method for this (with factors such as environment of use, load and age of the installation), but does not prescribe a fixed statutory term. In practice, 1 to 5 years is common for fixed installations, and more frequently for portable electrical tools that are used intensively or under demanding conditions.

31. What does a NEN 3140 inspection consist of? Two components: a visual inspection (visible damage, correct marking, unobstructed access to emergency stops and switches) and an inspection by measurement and testing (insulation resistance, loop impedance, RCD operation). Both are needed — an installation can look fine visually while a measurement nevertheless reveals a defect, and vice versa.

32. How long is a written appointment (VP/VOP) valid, and what if it is not (or no longer) valid? The validity period is stated on the appointment form itself; 3 to 5 years is common, with periodic refresher instruction as a condition for renewal. If the appointment has expired, its scope has changed, or it does not cover the installation in question, you do not carry out the work. You report this to the work-responsible person and request an additional or temporary appointment, rather than continuing on your own authority.

33. What do you do if, during work, you encounter an unsafe situation that falls outside your own task? You immediately stop your own work insofar as it affects the situation, secure the surroundings if necessary, and report it directly to the work-responsible person or the installation-responsible person. You record the report in writing. You do not resolve the problem yourself if you have not been specifically appointed to do so — even if you would be technically competent to do so.

34. What is the difference between a load-break switch (lastscheider) and a disconnector (scheider)? A load-break switch may be operated under load: it is designed to interrupt the associated current, and in the open position has a guaranteed isolating distance that is visible or demonstrable. A disconnector (for example a fuse-link or a knife disconnector without load-switching capability) may only be operated when de-energised or unloaded — opening it under load can cause an arc flash.

35. Why is the sequence for connecting and disconnecting the PE conductor fixed: disconnect last, reconnect first? As long as PE is connected, any fault current path to earth remains available and the touch voltage on conductive parts remains limited. Disconnecting PE last and reconnecting it first ensures that this protective function remains active at every moment of the action, including while the other conductors are being connected or disconnected.

36. What is the danger of replacing a circuit breaker one-for-one with a different type, for example B16 with D16? The thermal value (16 A) stays the same, but the magnetic tripping current rises considerably (from 3–5× to 10–20× In). As a result, the required short-circuit current needed to trip within the permitted time also rises, and the maximum permissible loop impedance Zs falls accordingly. A cable that comfortably met the Zs requirement with a B circuit breaker may no longer meet it with a D circuit breaker — with the result that the disconnection time in the event of a fault is no longer achieved. Conversely (D to B), the Zs requirement is not a problem, but the circuit breaker may trip unwantedly on a normal inrush current, making the installation unreliable. Both directions therefore require a fresh assessment, not a blind replacement.

37. What does 1P+N mean on a diagram symbol, and why is that relevant when de-energising? 1P+N means that the circuit breaker or switch only switches the phase (1 pole) and not the neutral conductor — the N conductor remains continuously connected. When de-energising such a circuit, you need to know this: the phase is interrupted, but the neutral conductor may still carry potential voltage via another, still-active circuit sharing the same N rail, which is relevant when demonstrating the absence of voltage between all conductors.

38. What is the danger of cumulative leakage current with multiple circuits behind a single RCD? Every connected device and every long cable has a small, normal leakage current (for example through suppression capacitors). With enough circuits behind a single RCD, the accumulated leakage current can reach a significant portion of the tripping value, causing the RCD to trip unnecessarily under an otherwise normal load. Rule of thumb: keep the cumulative leakage current below 30% of I∆n; if that is not achievable, split the circuits across multiple RCDs or use a type with a higher tripping value.

39. What is the difference between direct and indirect contact? Direct contact is touching a part that is live under normal operating conditions (for example a bare conductor core). Indirect contact is touching a conductive part that is normally not live, but which has become live as a result of an insulation fault (for example the metal enclosure of an appliance). Basic protection is aimed at preventing direct contact, fault protection at preventing danger from indirect contact.

40. Why do some practice setups operate at a reduced training voltage instead of full mains voltage? To eliminate the risk of injury while practising measuring and switching actions, while keeping the relationships between the phases (and thus the measurement logic) the same. A downside is that some measuring instruments — a voltage tester/duspol, an installation tester or a mechanical phase-rotation test plug — only give a reliable reading at full mains voltage (230/400 V); at reduced voltage they may show no reading, or an unreliable one. This is itself relevant practical knowledge: knowing which instrument still works at which voltage is part of professional competence.