Prepaid Meter Symbol
Definition: The Prepaid Meter symbol represents a pay-before-use kWh energy meter with an integral supply switch — drawn as a meter rectangle with a digital display band and keypad dots — with Line In, Load Out, and Neutral terminals, metering to IEC 62052/62053 accuracy classes and token handling per the IEC 62055 (STS) prepayment standards.
Also known as: prepayment meter, pay-as-you-go meter, token meter, STS meter, keypad meter, prepaid electricity meter, PAYG electricity meter.
What the Prepaid Meter symbol means
The Prepaid Meter symbol denotes a revenue energy meter that dispenses electricity against purchased credit instead of billing after use. The customer buys a credit token — classically a 20-digit number entered on the meter's keypad, or delivered over a smart-meter channel — and the meter adds the encoded kilowatt-hours to its balance. An internal latching contactor keeps the Load Out terminal energized while credit remains; when the balance reaches zero (after warnings and, in most utilities, a friendly-hours grace policy), the contactor opens and disconnects the load side until new credit is entered.
Electrically, the meter sits in series with the service: utility supply lands on Line In, the customer's installation feeds from Load Out, and Neutral passes through (and is monitored — reverse and earth-tamper detection are standard). The dominant token scheme is STS (Standard Transfer Specification, standardized as IEC 62055-41), a cryptographic one-time-token system used by hundreds of utilities across Africa, Asia, and Latin America; the token encodes the kWh amount, the specific meter's key, and anti-replay data, so a token works once and only on its meter.
How to identify the Prepaid Meter symbol
In diagrams the prepaid meter appears as a rectangle with a horizontal band representing the LCD display and a small grid of dots for the keypad — the two features that distinguish it from a plain kWh meter symbol, which shows only the display or a circled 'kWh'. Terminals are drawn bottom or side: Line In from the utility cutout/fuse, Load Out to the main switch or distribution board, and Neutral through. Split-type meters add a second box (the customer interface unit, CIU) linked by a communication line — the keypad lives on the CIU indoors while the metering element sits in a sealed enclosure or pole box.
IEC-style single-line diagrams place the meter symbol between the service fuse and the main isolator/breaker with the standard kWh integrating-meter glyph (IEC 60617's meter symbol annotated 'kWh') plus a 'prepayment' label; North American-style residential drawings rarely feature prepaid meters (utility credit metering is post-paid via the meter base), so the symbol appears mostly in international/off-grid and landlord submetering contexts.
Function in a circuit
Internally the meter chains four blocks: measurement (voltage and current sensing feeding an energy-metering IC accurate to class 1 or better), a microcontroller holding the credit register and STS decryption keys, the keypad/display interface, and a 100 A-class latching contactor in the Line-to-Load path. Every kWh delivered decrements the balance; programmable thresholds trigger low-credit beeps and display warnings. On zero balance the contactor opens; entering a valid token closes it again. Tamper detection (cover open, reverse energy, earth-return current, magnetic interference) can lock the meter into a tamper state requiring a utility reset token.
In the installation, the meter is wired like any series service meter: utility fuse → Line In, Load Out → main isolator → distribution board, with Neutral through the meter for measurement and tamper detection. Split (two-part) prepaid meters put the measuring/disconnect unit at the pole top or in a communal kiosk — out of tamper reach — and give the customer only the CIU keypad over PLC or RF, which is the configuration most utilities now prefer for loss reduction.
Standards: IEC vs ANSI
| IEC 60617 | IEC 62055-31 specifies particular requirements for payment meters (including the disconnect switch classes); IEC 62055-41 standardizes STS token transfer (the 20-digit cryptographic token); IEC 62055-51/52 cover the physical/protocol layers for split-meter CIUs. Measurement accuracy follows IEC 62053-21 (class 1/2 active energy) on IEC 62052-11 general requirements. STS key management runs through the STS Association's key management centre scheme; the 2024 'TID rollover' (token identifier epoch reset) was a global STS maintenance event. |
|---|---|
| ANSI/IEEE 315 | ANSI-market (North American) metering is dominated by post-paid socket meters per ANSI C12.1/C12.20 (accuracy classes 0.5/0.2) in ANSI C12.10 socket form factors; there is no widely deployed ANSI prepayment token standard — US prepaid programs (e.g. utility pay-as-you-go schemes) implement prepayment as a billing/AMI software layer over standard smart meters with remote-disconnect switches rather than STS tokens. |
| Key difference | The IEC/STS world builds prepayment into the meter itself — cryptographic tokens per IEC 62055-41, keypad entry, integral contactor — whereas the ANSI world implements prepayment in the head-end software of an AMI network, using the smart meter's remote disconnect under utility command. Physically, IEC meters are DIN/BS mounting with terminal blocks (Line In/Load Out/Neutral), while ANSI meters plug into a socket (meter base) with blade jaws. |
Terminals / pins
| Pin | Name |
|---|---|
| line | Line In |
| load | Load Out |
| neutral | Neutral |
Typical values
Single-phase prepaid meters are commonly rated 230 V, 50 Hz, with current ratings written as basic/maximum — typically 5(60) A, 10(80) A, or 20(100) A — and an integral disconnect contactor rated to the maximum current (100 A class). Accuracy is IEC class 1 (±1%) for residential revenue metering. Tokens are 20-digit STS numbers; credit registers count in kWh to two decimals. Three-phase variants run 3×230/400 V at 100 A per phase. Split-meter CIUs communicate over power-line carrier or RF and run from a small internal supply; meter standby draw is on the order of 1–2 W.
Where the Prepaid Meter symbol is used
- Utility residential connections in prepayment-first markets (South Africa, Nigeria, Kenya, India, Indonesia and many others) between the service fuse and the customer's distribution board
- Landlord and sub-metering installations — rental units, backyard dwellings, and flats where the owner resells electricity per tenant
- Informal-settlement and rural electrification projects using split meters with pole-top measuring units to curb tampering
- Student accommodation, holiday lets, and caravan parks where per-occupant energy accountability is wanted
- Small commercial tenants inside a larger metered property, fed from the landlord's board through a prepaid submeter
- Off-grid and minigrid systems where community operators sell stored solar energy by token
Example
In a single-phase service diagram, the utility's 60 A cutout fuse feeds the prepaid meter's Line In terminal; the meter's Load Out terminal feeds the main isolator of the customer's distribution board, and Neutral runs through the meter's neutral terminals for measurement and tamper sensing. The meter is a 230 V, 20(100) A, class 1 STS unit: the occupant buys a 50 kWh token, keys the 20 digits on the keypad, the display credits 50.0 kWh, and the internal 100 A latching contactor holds Load Out live until the register runs down.
Key facts
- A prepaid meter is a revenue kWh meter plus a credit register plus an integral latching contactor — it disconnects the load side automatically when credit reaches zero.
- STS (Standard Transfer Specification, IEC 62055-41) is the dominant token system: a 20-digit cryptographic one-time token bound to one specific meter, used by 500+ utilities worldwide.
- Terminals are series-service style: Line In from the utility fuse, Load Out to the customer's main switch, Neutral through the meter for measurement and tamper detection.
- Typical single-phase ratings: 230 V, 20(100) A (basic/maximum current notation), IEC class 1 accuracy, 100 A-class internal disconnect.
- Split (two-part) meters separate the sealed measuring/disconnect unit (pole-top or kiosk) from the indoor customer interface unit, communicating by PLC or RF — the standard anti-tamper architecture.
- Tamper detection covers cover-open, reverse energy, earth-return (bypass) current, and magnetic interference; a tripped tamper state needs a utility reset token.
- North American utilities implement prepayment differently — as AMI software plus the smart meter's remote-disconnect switch under ANSI C12 metering — rather than STS keypad tokens.
- The global STS TID rollover of 2024 required every STS meter to be re-keyed with two rollover tokens — a reminder that prepaid meters are cryptographic devices with key lifecycles.
Frequently asked questions
How does a prepaid electricity meter work?
You buy credit from the utility or a vending agent and receive a 20-digit token cryptographically bound to your meter. Entering it on the keypad adds the purchased kWh to the meter's balance. The meter measures consumption like any kWh meter and counts the balance down; at zero (after low-credit warnings), an internal contactor disconnects your supply until you load a new token.
What do the Line and Load terminals mean on a prepaid meter?
Line In is the utility side — fed from the service cutout or fuse and always energized. Load Out is the customer side, feeding the main switch and distribution board, and is only energized while the meter has credit and its internal contactor is closed. Swapping them is a serious wiring fault: the meter's disconnect and tamper logic assume power flows Line to Load, and reverse energy registers as tampering.
What is an STS token?
STS (Standard Transfer Specification, standardized as IEC 62055-41) is the global prepayment token system. A token is a 20-digit number encoding the credit amount, encrypted with a key unique to your meter, with anti-replay data so it can only be used once. Because the cryptography binds tokens to one meter, a neighbour's token will not work on yours. Utility 'engineering tokens' from the same system perform resets and key changes.
Why is my prepaid meter not accepting my token?
Common causes: the token was vended for a different meter number (check the meter serial against the receipt), the token was already used, the meter is in a tamper lock state (needs a utility reset token), or the meter's key revision no longer matches the vending system — for example if key-change or TID rollover tokens were never entered. If the display shows a tamper or error code, the utility must issue the corrective token.
What does 20(100)A mean on a meter nameplate?
It is the IEC basic/maximum current notation: 20 A is the basic (reference) current at which accuracy is specified, and 100 A is the maximum continuous current the meter — including its internal disconnect contactor — can carry. A 20(100) A meter suits a typical 60–80 A residential supply. Exceeding the maximum rating overheats the meter's current circuit and contactor.
What is the difference between a prepaid meter and a smart meter?
A prepaid meter enforces pay-before-use with an onboard credit register and disconnect, traditionally via keypad tokens and no network at all. A smart meter is a communicating meter (AMI) that reports readings remotely and may include a remote-disconnect switch; it is payment-agnostic. The categories now overlap: many modern meters are both — STS-capable prepayment meters with AMI communications, or smart meters running prepayment as a utility software service.
Related symbols
- Circuit Breaker symbol
- Disconnect symbol
- Energy Meter (kWh) symbol
- Main Panel symbol
- Meter Base symbol
- Service Entrance symbol
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