Summary
Why?
A meter is not a protective device. It therefore allows short circuits to pass through, and it is the upstream circuit breaker’s role to trip in order to protect the installation.
What protection should be implemented according to the meter’s rated current?
The rating of the upstream circuit breaker must be determined according to the load, without exceeding the meter’s rated current.
Example: for a 100 A meter, the maximum rating of the upstream circuit breaker should be 100 A.
It is common practice to use circuit breaker ratings lower than the meter’s nominal current (for example, using a 63 A breaker with a 100 A direct meter). In this case, the meter is fully protected.
Conversely, if the protection exceeds the meter’s rated current, a permanent overload above the meter’s rating could cause an incident in the installation.
The table below shows the maximum rating for each type of meter
| Pulse energy meters | Pulse energy meters | Modbus energy meters | Modbus energy meters | Modbus MID energy meters | Modbus MID energy meters |
|---|---|---|---|---|---|
| EM145PU | 45 A | EM140PURS | 40 A | EM145PURSM | 45 A |
| EM180PU | 80 A | EM180PURS | 80 A | EM1100PURSM | 100 A |
| EM380PU | 80 A | EM380PURS | 80 A | EM3100PURSM2 | 100 A |
| EM3100PU | 100 A | EM3100PURS | 100 A | EM3100RSDTM2 | 100 A |
What happens in the event of a short circuit downstream of the meter?
To ensure their quality, energy meters are manufactured according to the EN 62053-21:2003-06 standard. This standard requires that meters be able to withstand a short-duration overcurrent of 30 Imax with a relative tolerance of +0% to -10% for half a cycle at the rated frequency.
For example, on a 100 A direct meter:
- If the maximum current (Imax) is 100 A, then according to the standard, the meter must withstand 30 × 100 A = 3000 A for a duration defined as below.
- The network frequency is 50 Hz. The meter must withstand the above current for half a cycle at the rated frequency.
- Since F = 1/T, then T = 1/F. Therefore, T = 1/50 = 0.02 second
- Thus, for half a cycle at the rated frequency, the duration is 0.01 second.
Therefore, a 100 A meter can withstand a short circuit of 3000 A for 0.01 second.
In conclusion, it is necessary that the upstream circuit breaker interrupts the installation before reaching this value.
Reminder of technical terms
Permanent overload: a situation in which the current exceeds the meter’s rated capacity continuously or for a prolonged period without interruption, which can lead to overheating, premature aging, or even damage to the meter.
An overload is a type of overcurrent.
Be careful not to confuse these terms with
Overcurrent: a situation in which the current exceeds what the system is designed to handle. It can be:
- temporary (or transient): brief, such as during a motor start-up,
- prolonged (or permanent): continuous, like an overload,
- very high and instantaneous: as in the
case of a short circuit.